Son playing on IPad instead of doing homework
Without installing additional applications (since is not your iPad after all), you could use the Battery consumption indicator that comes with iOS as a rough indicator how long a app was running- consuming battery. It does not tell you the time of usage but the amount.

The benefit is you will not be intrusive nor raise any suspicion from the "user" looking at the battery consumption, as compared to some tracking software, since the kids are very clever these days.

iOS 8 tells you what app is draining your iPhone's battery

Example for 7 days (Clash of Clans is a game) and here you can see the heavy usage over a week that might help you managing the situation.