I just ask my kid for ideas, and he tells me! I tell him what we have that needs to be used up, or what ingredients are on sale this week, and he tells me what he wants us to make with it so that I know what other ingredients to get. Sometimes he looks through our recipe binder
for ideas. But that's him planning the menu with me, for dinners that his dad or I will cook and he might help. Breakfasts in our home, and most lunches, are just spontaneous using what's on hand that you feel like eating. The only time he has done meal planning and cooking completely on his own was the first weekend we had the new baby and no grandma visiting: He decided what to make for each lunch and dinner, using stuff we had on hand, and made it. We keep a lot of basic foods in the house and had bought more convenience foods (mac&cheese, frozen veggie burgers, canned baked beans, frozen ravioli) than usual, so it was pretty easy to do.
My mom got very serious about my brother and me taking responsibility for lunches the summer we were 6 1/2 and 9 respectively. She sat down with us to brainstorm a list of what kinds of things we liked to have for lunch; this list stayed on the refrigerator for years, and we added to it. Then it was our job to work out who would cook each day and to each write down our menus for the upcoming week by Sunday night, and she would make sure we had the ingredients, at least approximately. (For instance, one thing we liked to make was custom salads for each person. Instead of our making a list of ingredients for the salads, we would write "salads" and she'd buy whatever veggies were on sale if we didn't have enough from the garden to have an array of raw veggies available, and make sure we had some kind of protein that would be good on salad.) It worked really well...but we were kids who focused easily and liked to have responsibilities. Your mileage may vary.