How do I find an app that got lost following upgrade to iO6
New iOS versions never break compatibility with older apps. You could try viewing your purchases in iTunes on your computer. This could be fruitful as the app may have been hidden — to check, go to your account details page (in iTunes) and select "View Hidden Purchases."

Also, consider the possibility that it was downloaded with an Apple ID you haven't thought of. If you're certain it's not on the iPad, you can't find it under Purchased, and neither you nor your kids can remember its name, then it may have been pulled from the App Store. This isn't very likely, though.