It's not that they're better-off, but rather that the movie theater discovered that a movie the child wants to see often forces the parents with them. By discounting the child, they increased the sale of at least 1 ticket for the parent, and therefore, they make more money. If the child wasn't discounted, the parent might choose an alternative source of entertainment (because face it, adults tend not to want to watch children movies) and the theater lose out on that adult ticket!

this is why you don't see a discount for couples usually - coz they were going to pay even without the discount.