That would be an option to ask the parents to setup a Google Account. I guess you will be using the servers of MIT else you could run a local version of App Inventor. Then you can use ficticious names for the accounts.
You can download a version of App Inventor from here that is synced with the version from january.
Or you can setup your own server with the up to date sourcecode
The following is an idea one of the PowerUsers Hossein once had:
"Just a thought: Since this is a workshop and their activities will be monitored, maybe a series of accounts could be created by you (e.g. summerworkshop1, 2, 3, ....). Before session starts, log everyone in so they don't have to go through login process. "
Maybe that could be an option.