I'd recommend keeping your eye on Craigslist, too. Bows are another one of those kid's items that they try, and sometimes just don't take to, long-term. So oftentimes you can find them like new for a fraction of the price. The classifieds here would of course be another place.
I think pretty much all of the major brands have a kids bow. Two last points to keep in mind... 1)Make sure it's something they can pull without looking like it's a feat of strength. 2)Once you have the bow, go to a good shop and get some properly-fit arrows. The performance between an arrow designed for a very lightweight bow, and one that isn't, is pretty remarkable.
In short, make sure they have what they need to shoot to the best of their ability. And most importantly, enjoy the time spent with them. My boys are 13 and 15, and it does just fly by!