First of all I would partition the users and take away admin rights on your children's accounts, only you should have this. Then I'd buy 2 lots of software, a filtering one and a monitoring one. The filter should prevent access to undesirable sites and the monitoring will enable you to view exactly what your children are doing. Monitoring software may be a little controversial but you don't have to tell them about it and my view is they are kids and you need to protect them. Hth