Never allow a children below 12 access to internet, a not give to they cellphones until the 14 or until they needed, in my experience as father, I give him 4 or 5 hour in videogames or videos in the computer monday-saturday, sunday is for family time, everytime he ask for buy something, I tell him that he must work for obtain that as reward or compensation, not let them take the monitor outside of your range, even in his room my son has the monitor close the wall but in front of the door so I can see what is he doing when I pass to his door, our family policy is that all the rooms are with open doors, is forbiden stay in a closed door, constantly I ask what is he doing in the computer, sometimes he lies but is ok, I not cross certain limit, sometimes I stay with him looking him playing his videogames, talk a lot with them, tell all the good and bad things that are in the internet, tell about the scamers, and all the bad actors in the social networks, but most important, show they that you love them and you care them and those are the reasons why are you doing this things.