I am looking for suggestions for an app for a MAC/IOS device that will allow my kiddos to use their computer/iPads but not go to specific sites without my permission.

Perferably a FREE app, but I am willing to pay for a really good one.

Whether it be specific times of the day or password driven it doesn't matter. I only want to know of ones that you have worked with personally and they actually serve this purpose.

My kids sometimes get sidetracked and end up on other sites, playing games or watching cat videos etc. instead of working on homework.

I want to be able to allow them free access to their computers without watching them like a hawk 100% of the time, but also make sure they get their work done first.