As a parent myself, screen-based devices are a class all to themselves. While earlier generations were worried about kids overconsuming things like radio, comic books, television, arcade videogames, etc... devices like the iPad are way more addicting. Perhaps exponentially so.
Talk to any parent and you'll hear stories about how they put their kids to bed and discovered, later on, that the kid woke back up and stayed up until 4am watching videos and playing games.
One way to fix that is to remove the device. But we're teaching self-control and trust, right? So what do you do?
But self control is just a skill kids need to learn by itself, if they have it as a general ability then they can apply it to screens too. Our kids have to do self-study time on top of homework, they do piano and flute, we take them horse riding and skating. They just joined the local Air Cadet squadron and just did a weekend camping. Spend time with them, do things together, encourage a variety of activities, talk about issues and build close, trusting relationships. I don't think intrusive surveillance has a place in that recipe.