The issue with his ordering from the children's menu is that you are paying for an adult plan. Which means you lose some money on that plan for every children's meal he orders and you pay for with an adult dining credit. You may not care about that, but if you do, then you can pay for the children's meals you order for him and use the credits for extra meals for the rest of your group.
If you had plans to do buffets, family style AYCE or fixed price/dinner shows, then you'd be paying the adult price for him no matter what he ate, so it wouldn't matter so much.