Just a thought: You might also get some ideas by looking at websites or cookbooks that focus on entertaining and offer “make-ahead” recipes where you assemble everything beforehand, and then just pop it in the oven for cooking or reheating. Or this looks promising:
For such small children, I would also think in terms of small meatballs, fish sticks and other child sized bites. Children often enjoy stews or baked dishes, and even salads, that incorporate fruit (like apricots, pineapple, raisins, apples), and its healthy. Also, unless you live someplace that is relentlessly cold, maybe a few meals that are no-cook-at-all would be welcome, like a buffet platter of sliced meats and cheeses and crunchy veg, dip and crackers that can just be unwrapped and set on the table.
I would also encourage the chemo patient to give you very honest feedback on what foods are really working for them personally, and see if you can make sure they always have that food on hand even if the rest of the family is eating something else. Sometimes when people feel unwell, they really only want to eat the same simple soup every day or a milk-based dish like rice pudding. I’m sure the parents want to make sure the kids keep eating healthily during this period, but the focus should be on making sure the patient is eating, and if the kids end up eating a bit more take out or nachos than they otherwise would be allowed, it’s less important.