CB, the best way to find premium foods is to look for non-chain or franchise pet shops. Many of them are attached to grooming salons. Stay away from the foods sold in vet clinics like Science Diet. They are crap.
Go into the shops and explain that you have a growing kitten, and describe Lily's skin problems and lowered immune system. They should be able to recommend a food that meets your cats' needs.
You will likely find that they recommend a grain-free diet, which is what I recommend as well. Nearly ALL cats do best on grain-free.
I have an odd-ball in that Thomas, who has what is apparently mild colitis, actually does better on a diet with some grain in it. My vet recommended switching him from Orjen to a Blue formulation that contains rice, barley, and oatmeal. This is slowly clearing up his chronic diarrhea.
This is not the norm, and I highly recommend feeding your cats a grain-free ration. Orjen and Acana are probably the best out there, but are expensive.
Any way you look at it, until your cat transition over and are used to the new food and have gotten their nutritional deficits caught up, you will be spending more on cat food. You should also drop the friskies canned food. If they are drinking enough water, they won't need it with the premium dry food and Friskies is crap. If you want to feed canned food, split a 3 oz can of Fancy Feast, which is at least mostly meat, between the cats at night.
If you go the grain-free route, you do not need to feed kitten food to the baby. Just let him eat as much as he can hold several times a day, or leave the food out in a dispenser for the cats and let them free-feed. Make sure there is plenty of fresh,clean water for them at all times.
If you want to go whole-hog, there are plenty of canned premium rations available as well, and even soft-moist premium rations. I feed a soft-moist premium ration as a substitute for canned food in the evenings. I give each cat a small handful of the stuff as a treat. They have Blue down in a dispenser for free-feeding. The soft-moist food is too expensive to feed as a stand-alone ration for me, though the cats love it.
Do NOT feed grocery store soft-moist food! It contains some ingredients that are actually toxic to cats.
hth
toK