Securing a young child's tablet
The overall outlook is not so great for you, AFAIK.

The Market has content filtering, check the Settings in the Market app.

For the browser, that's a wholly different kettle of fish.  The built-in browser doesn't have any kind of filtering available.  3rd party browsers might, but they'll be trivial to sidestep (by simply opening the built-in browser).

The next best thing you can do is filter the tablets at the router, if it allows for that.  However, that's only useful when they're at home (or other places where you can control their internet connection).