Auto Lock Screen after 30min - hard rule - for managing kids usage

Hi,

I'm new to Apple, yes, I have never had anything Apple in my 30 years of life. Until recently, I bought an iPad Air so my son won't keep taking my mobile phone which has tiny screen and I worry it's not good for his eyes.

Long story short, I'm looking for a function/application that can automatically lock the screen after certain time, e.g. 30 minutes, regardless what apps are running or what so ever. I need this to control how much time my son can use the pad each time. At the moment, I try to keep an eye on the clock, but when I'm busy with housework, I lose the track. Plus, he knows I would be after him, so he hides himself when using the pad.

Any suggestion? I don't see anything obvious in the Settings though.

Thanks in advance.