Is it legal for a restaurant to refuse to serve an item from the children's menu to an adult?

As long as the restaurant is not breaking any civil rights laws, sure, but it doesn't sound like a very good business practice.

A Different Perspective

Yes, it is legal as well as reasonable. Children's items on a menu are added as a courtesy to the patrons who wish to have their children accompany them for dinner. The restaurant owners recognize the need for smaller children's portions at a much reduced price and also recognize that creating an environment friendly to families will increase business. The restaurant couldn't stay in business very long if it served adults from the children's menu. The profits wouldn't be high enough to sustain the cost of all the service required to serve the meal at the restaurant.