How long ago did your kids buy Pro? Pro entitles you to free updates for up to 6 months (it's up to the user to save & back up their Pro installers). So if the Pro was bought more than 6 months ago the email address won't be in the database anyway. In that case you'll need to pay again if you want Pro.

If Pro was bought less than 6 months ago, go to this page...
http://sales.limewire.com/support/pro_lookup.php
...& try all of your email addresses