I'm on a pretty tight budget as well, but its just me (making food choices for one is a lot easier than 5!) but I do have one suggestion from my personal experience:
Don't try to make substitutes for cookies, brownies, cake, etc. Just drop them out. Like you pointed out, the alternatives for flour are really hard to justify on a tight budget, since you can't buy much. I also read somewhere recently (I can't remember where, so if someone can give credit, please do!) that its not always a good choice to eat whatever the substitute is (ex. the amount of almond flour needed to make cookies turns out to be a lot of almonds!)
For treats/desserts, I've made my own version of a coconut oil ball. Its small and sweet and filling, so hopefully it will work on the little ones! I just melt some coconut oil and honey and throw whatever extract I have in my cabinet. I tried peppermint last time, and they were delicious! (I'm so poor that I had to reuse a styrofoam egg carton for this next step!) Separate the liquid mixture into bite-sized amounts and throw them in the refrigerator to set.
I hope this helped a bit!