My kids eat cereal with no more than 10g sugar every morning and maybe fruit, yogurt, or a boiled egg.
They bring their lunch and take either a sandwhich with no sugar added bread, real peanut butter, and 100% fruit jelly or natural turkey(no nitrates) and cheese sandwich, 2 snacks (no sugar added applesauce, fruit, crackers, fruit ropes, cheese sticks --stuff like that) and a juice box. Once in a long while they take Ramen with organic all-purpose seasoning(I don't let them use the seasoning it comes with)in a thermos or a thermos of spaghettios.
I have 4 kids and 2 are adopted with rough early childhoods. i have never had a lick of trouble with any of them in school with regard to behavior, ability to sit still and focus, or being too tired at any point throughout the day, unless of course they had a string of days when they stayed up too late. My personal belief is that it has a lot to do with the fact that they eat very little sugar on a regular basis and very little food with neuro excitotoxins. I hope this gives you some ideas.
