How do I put "parental control" in our laptop/tablet/cellphone so that the kids can only access child-friendly apps/sites?

It really all comes down to what device you're using. Obviously, each device has different functionality and ways to adjust them...in any case, if you use Google Chrome, this video will show you all that you need to protect your laptop and maybe even phone if they have Google Chrome as their browser: https://www.youtube.com/watch?v=nfeC47ixmCs