I'm almost 50 and I ate off the children's menu a few times two weeks ago. As PP pointed out, you'll pay adult prices at buffets and all you care to eat type meals.
Another alternative if the child just isn't a big eater but wants more variety than the kids meals is to order an appetizer as an entree - I do that sometimes. Or splitting a meal is perfectly acceptable. DD and I have split entrees when we wanted to make sure we have room for dessert.