Lots of parents seem to check their kids social media. But lots of kids also do things to make these checks invalid. For example I have just read a teenager who has been advised by another teenager to create a second instagram account that is their real account, and post inane stuff on the first account their parent checks. And snapchat, telegram, wickr were invented for this reason, so that messages are deleted after they have been read and parents can not check what their kids are saying.
So do you really think you will know what your kids are up to on social media if you check their phones regularly?