Parental control apps


I've bought a new Nexus 7 for my son for Christmas and I'm looking for a good parental control app to restrict his access to only apps and features that we have approved. There seem to be a lot of these kind of apps out there but many seem to assume the tablet is shared and require the parent to activate them each time the tablet is started up.

As this tablet is going to be for my son's sole use and he will want to pick up and play, are there any (good) parental control apps which start up automatically when the tablet is switched on?

Thanks for your help.