Thanks to Mud Pie's 2nd grade teacher, I discovered Symbaloo. Which is really nothing more than a cross-user, cross-platform version of Safari's "Top Sites" start page. Well, ok, it is more than Top Sites, because with Symbaloo I can make a webmix (or mix of bookmarked sites - that's visual rather than a pulldown list), and I can put it as the homepage for each of the kiddos. What's more, they can each have their own customized homepage!
I can put Dance Mat Typing and Starfall on Mud Pie's, Spelling City and TimeMaps on Fish's, and National Geographic and Kitchen Chemistry on St. Nick's. Along with about three dozen more - each. Each kiddo can have a customized background, too. Even Rowdy has a home page, though there's not much on it since she'd rather eat the mouse than click it.