Wow! My problem is the exact opposite of yours!
I have an 11yr old who is a small eater and prefers the kids meals!!!
If you eat at a buffet, you will be fine. He can go up as many times as he wants and even pick from the adult section, I dont anyone really notices/cares.
If you do order from a CS or TS and it’s a la carte, he will have to order from the kids meals if that is what you bought for him (kids meal on DDP).
They lump all the adult credits together and the child too but will be listed separately on each receipt.
But if you order an adult meal for him, they will deduct that from your allotment of adult meals that is left.