Think, How convenient or uplifting is that you allow young children to use a social network? -you Have the answer. His duty as a parent is trying to preserve their children from the influence of the world on them, not to expose them to it.
Second, Do you really know your children / as, you know who walks, who is the boyfriend or girlfriend? -¡Fácil !, Ask him to accept as friends on the social network that he or she uses and see that things are stuck. If you do not understand the world of internet or can not create an account ask your child / to occasionally teach you everything he or she shares with others on the Internet, if he can do that
In conclusion:
-The Use the internet or a social network is not sin in itself, the problem lies in the use we make of it, that may well be to bless, to share and have fun healthily with others, even to bring the gospel to our friends or While it may be a weakness of Satan to take to make you look bad to the kingdom of God ... you decide.