A few years ago I was asked by a concerned parent if I could limit their child's experience of the internet. They didnt want them accessing frivolous websites during the time they were supposed to be doing their homework. The child had got round every security measure they could think of.
I managed to make it so that even if the router was reset, the parent would still be able to read every email to and from the child, see everything the child was seeing - monitor everything that child was doing online. the olly way they could get around it would be to reinstall the whole operating system AND reset the router and even then the parent would be notified
if you really want to monitor your children
spend some money consulting an IT network expert (dont call me though I dont do that no more)
it should cost you a few hundred pounds but it's worth it