Mobile devices are terrible to police.
Constant war in my house over the mobiles.
This is techy stuff but you can limit the strength of the wifi signal,
list the phone I.P. as blocked on the router, block sites etc.
Try digging around the net.
http://content.mobicip.com/content/how-setup-parental-controls-iphone-ipod-touch
I monitor my kids by randomly taking the phone and looking at it. Once they know I do that, they tend to behave.
I don't give a hoot about privacy - I am more concerned about their safety.
I also do not allow pc's upstairs for any reason.
They also do not have the internet password.
Sure they can surf if they have credit but its slow and harder to get upto mischief.
If you want total control over all your kids access and internet - Pc's are easier to block.
You can also put logging software on a pc which shows
exactly where and what anyone does on the net.
Over the top big brother I hear you say?
Unfortunately the underbelly of the internet still scares me and I'm on it 20 odd years.
It's never been easier for kids to access things they shouldn't on the internet.
It's my job to protect my kids from it.