You need to watch the fat intake, the salt and the sugar. That means - avoid fast food. There is only one remotely healthy fast food and that's sushi.
Suggestions - you have summer coming in, so get fresh fruit and freeze it. You can then puree the fruit, frozen, and serve it in a cup with a spoon. It's like a thick shake, really delicious, loads of fibre, thirst-quenching, no added sugar (and you shouldn't need Splenda, either). If they want it sweeter - add a frozen banana.
Or you can make it with fresh fruit. Banana sweetens it, especially if the bananas are over-ripe and beginning to get black spots on the skin.
Fruit can be eaten frozen - strawberries, bananas, raspberries (or other berries), mango, passionfruit (freeze the passionfruit whole, then to use just cut them open while frozen and scoop out the ice block that has formed inside), pineapple, oranges (don't puree oranges with peel on). All of these can be eaten frozen. On my diet - my current dessert is a pure fruit juice (mango nectar) with no added sugar (although it has added mango pulp) which is frozen, I eat it like an ice block. Yum! Watch out for brain freeze, though.
Oranges can be cut into segments and frozen, kids can then eat them as they are using the peel to hold it.
Grapes are yummy when frozen but don't try to puree them - the skin is a bit much. But eaten frozen - wonderful.
You do need to be careful to not over-eat with fruit, but for kids overindulgence in fruit is far less of a problem than for us adults.
I have a banana cake recipe to die for, because I follow the old rule - you can substitute up to half the plain flour with cocoa (I substitute about quarter to a third). If you've using self-raising flour, you need to add baking powder to compensate for the loss of leavening in the flour you've substituted.
When out and about with the kids and they're nagging for you to buy them lunch - if they're not into sushi, the OCCASIONAL fast food is not so bad, but more than once a week for a kid who is NOT overweight is asking for trouble. So our alternative - we go into the supermarket deli section (or just go to a deli) and buy whatever sliced cheese or sliced meat they want. Smoked salmon, even. Only buy just enough for a generous serve for lunch, you don't want to be dealing with leftovers. I often buy a small pot of low-fat cream cheese to use as a spread instead of butter.
Then go buy some fresh-baked bread rolls. Have fun, get something fancy if you want, or plain if you prefer. Again, it can be made individual, from the hot bread shop.
Then go into the greengrocer's and get either some mixed salad greens, or lettuce, or tomatoes - whatever you need for your vegetable shopping anyway.
Then depending on how demanding the kids are, either find a comfy chair in the mall (we have them in a lot of Aussie malls) or if you've finished your shopping, load everyone into the car and find a quiet spot by the river somewhere, and have a picnic. The food tastes so much better outdoors and away from all the noise.
I call this the family version of Subway. It's healthy, we can make it individual, and it's all really freshly made. It's also vastly cheaper, even with gourmet ingredients.
Another trick - give your kid expensive tastes. As our kids have grown up and begun drinking alcohol (legally), we've worked to get them used to vodka and freshly squeezed orange juice, instead of those ghastly ready-mixed drinks of cheap alcohol and cheap orange-ade sold in a can or bottle. Alco-pops. Hate 'em. Instead, the kids are shown how to put a small amount of vodka in the freezer (because a large bottle won't fit) and a bag of oranges, for much more enjoyable drinks. So next time they go to a party - they won't enjoy the cheap and nasty stuff, and so are less likely to over-indulge.
My favourite for slowing down a young drinker - vodka, freshly squeezed lime, teaspoon of sugar. Mix it all thoroughly, then put it in the freezer until it's a slurry. Great for midsummer. You can't overindulge because you have to wait for each one to freeze!
After school snacks - vegetable sticks with some dips. I make an easy dip with cottage cheese and corn relish. Mix together in proportions to your liking, serve with vegetable sticks. The first time I had this was with cream cheese which needed to be watered a bit to make it spreadable. Really yummy, husband & I ate a vast amount. My sister in law made it for the boys after a day's sailing on Lake Macquarie in my brother's 40' yacht. As we tucked into the dip, all thoughts of "Captain Bligh, we will get you," faded. I found myself liking my brother again, all because his wife had made a delicious dip.
A platter of fruit pieces goes down well, too. I've found if you cut fruit up and peel it, kids will eat it. I use a melon baller to core apple halves. Or you can use it to core a whole apple, then stuff the apple with pecans, raisins, brown sugar (or maple syrup) with a dob of butter on top, then bake it. Do this with green apples, not red (Granny Smith apples are best - do you get those?). Maybe better for winter.
Splenda is good, as Psalm suggested. But make sure you mix it in well and only use it if they insist something is not sweet enough. Use it sparingly. I've found that over time, they lose their taste for sugar even if you use Splenda.
With drinks - we had a rule that every second drink had to be plain water - "water's turn". It's more of an individual habit to get into because it's very hard to police. It was the compromise I made so as not to water down their juice too much. We had a friend at church who is noted for her extremely weak cordial - we reckoned she just waved the lid over the top. Really weak drinks, when you want something with flavour, is worse than plain water sometimes. So having a small amount of something sweet and tasty, followed by the water you should have had with it, was our compromise for the sake of health.
You may find resistance if you make too many changes too soon, but if you can switch to a fruit smoothie as I described instead of a glass of cola for example, you might find them enjoying it.
My kids get the occasional glass of soft drink, but we also have to watch the caffeine content (bear it in mind). The caffeine free colas we can get are also sugar-free.
And that fruit smoothie of mine - you can freeze it into ice block moulds (put a stick in each one) and that is a good treat as well. You may need to individualise them - a couple of my kids hate banana in a drink, so I substitute mango - and to make it an intense deep pink, you only need 1-2 raspberries.
You can do milk ones too, with banana, maybe malted milk powder, a dash of chocolate powder if you want and a raw egg. When I've had kids unable to face breakfast, I've made this for them.
Breakfast - check the cereal, chances are it's high in sugar and sodium. I've switched carbs to high-fibre, so muesli is now our breakfast of choice. But muesli can also be individualised. Toasted muesli is also higher in fat & sugar, so watch it.
And remember, just because the ads tell you how healthy it is and good for kids - it ain't necessarily so. Things like muesli bars, yogurt bars, fruit straps, fruit bars - read the fine print. Sometimes the only fruit in it is the picture on the box.
What you cook yourself, you know what's in it. Anything else - pure guesswork. And you can be sure that if they're trying to make money, they'll pad it out with cheap carbs. I bought a rice paper roll from a health food bar thinking it would be good for me, full of sliced grilled chicken fillet, sprouts, lettuce and other good things - it was about 50% rice vermicelli. OK for kids, but not for me on a low carb diet.
In summary - you need the kids on side. For this, you need three things -
1) Education. They need to understand why this is better for them, and to really embrace it; and
2)Taste. Win them over by showing them how much more delicious it can be.
3) Moderation. There is always room for comfort food, let them know it's not goodbye forever to the junk. But junk in moderation, at the right time and for the right reasons. Never medicate with food - if a kid is unhappy, find another positive thing to do with them. For example, when easy child 2/difficult child 2 broke off with her first serious boyfriend and he really hurt her, we tried retail therapy. He had blogged all over the Internet that "she's got the maturity of a 12 year old and the boobs to match" so we bought her a couple of lovely lacy blouses which really emphasised her cleavage! A slab of cake would have been gone in minutes and she still would have felt miserable. But every time she wore one of those blouses, it was a mental thumbing of her nose to the ex.
I hope this helps.
One last thing - my sister in law just went on the CSIRO diet (an Aussie thing, very good). She cooked a meal from the diet book and served it to her family. Even though they're all thin and don't need to diet, they told her to go ahead and cook more, this stuff tastes great!
Marg