have the kids look online for copy cat recipes of the fast food they like. Then have them help you make them. Sounds like they are "addicted" to fast food and junk food. My son is. I stopped cooking for him and he has to make or buy his own.
Flower, I suspect CB will find that her kids have absolutely no interest in cooking "fast food" for themselves. This is a power struggle. The food thing is a symbol. Getting what they want to eat when they want to eat it, is getting the power in the relationship, and being able to hurt CB's feelings by insulting her cooking skills adds more pleasure to the win.
I think, CB, you need to do what a couple of other posters have recommended. Buy and cook for yourself. Have on hand peanut butter, jelly, tuna, and mayo/salad dressing, as well as some wholesome bread.
Either they eat what you cook, or they're welcome to make themselves a sandwich. Do not buy stuff they like, no junk or snacks. Just healthy, wholesome foods, including fruit and veggies. No chips or similar. Get some baked whole grain crackers to eat with the salads you are eating that they are welcome to make for themselves, or eat with the vegetable soup you've made in the crockpot for yourself.