Not only do kids get a checked bag allowance, it doesn't have to be used for "their" bags.
If it's two adults and two kids, and the checked bag allowance is 2 per person, the you can check 8 peices of luggage between you, regardless of who's bag it is. So if Mom has 3 peices, and little Timmy only has one, he can check Mom's extra peice as part of his allowance. Cool, huh?!