I have no idea. There is a decent variety of foods that are 1000 calories for a dollar, and maybe combining all of them together is enough variety. But I'm not a doctor.
More variety in your diet is likely to always be superior to less. That goes for both kids and adults. The trouble with younger kids is that deficiencies can impact their development and have more severe long term consequences, and they're also less capable of seeking out foods to fill that gap.
My mom basically starved so we could eat. I remember her giving me her food regularly.
I Still only ate once a day even with her sacrifices.
I expect I will be doing the same at this rate so I wanna do more for them if I cam.
If the option is you all not eating enough, and eating primarily cheap bulk foods, do pick the cheap bulk foods. With cheap seasonings and making your own tortillas, breads, and gravies, you can have a large variety using the same 20ish cheap base ingredients.
We are tortillas and rice all the time. Some days evey meal. Sure she upgraded it with veggies or added eggs but it's like I was born of rice, molded by it, I didn't know what a salad was until I was a man.
But I mean the main things you need are calories, vitamins, and complete proteins. Flour is the cheapest calorie you can get in the US, so cooking your own biscuits, tortillas, sour dough, and gravy will always be the most calories for your buck.
Soy is a complete protein by itself, but rice and beans together are as well. Rice and beans is also a king of calories per price, so there is a reason I put it on there, and a reason you are alive it sounds like it!
The last thing is vitamins from veggies/fruit. In my post I used small amounts of dried fruit in oatmeal, peas in gravy, then tomato sauce. So getting a mix cheap frozen/canned/dried fruit then having a bit of that each day will help. As will making gravies/sauces with different veggies/stir fries.
I was able to do it under $2, and I might try it again with all different meals to try and make sure I can practice what I preach. I make one off cheap meals a lot, but don't always do a full day.