Give it a password that only you are likely to know and then only visit that Account on special occasions.
Then log your own account in the same way as the kids. That bit over.
Not really is the answer to your question. There are a number of programs that will restrict surfing but are only useful with those that are under 10yo or over 50.
There are Key Loggers which will monitor their use of the net, but a friend who installed one said that he spent so long trying to follow what his 14yo was up to that he gave up after a couple of weeks.