So, I've been checking out Venuwize which is described as a wireless events dashboard letting event organizers collaborate, check in guests with iPads and iPhones, access live guest information, build a guest database and stay green. Sounds impressive, but let me just highlight some features. First, it's free if you have less than 100 guests for an event. The iPad or iPhone app required to check-in your kids is also free. It's free to sign up to trial the program, too. And it's pretty simple to set up. All you need to do, once you've signed up, is set up an event (and you can make it a recurring event) and then add your guest list. That is easily done by uploading an Excel file of the kids in your program. Extra children can be added individually, too, and you can add important information such as allergies or medication info as notes for particular children.
Once it's set up online, you can easily check-in the children using the app- just put in the code for your event and click on your guest list (list of children) and you're ready to check them in.
The only drawback that I can see at the moment, is that there are not extensive reports available about your events. But.. as a basic check-in system, it's worth taking a look.