Or another option. Create a new google account for your son (which you control and know the password for) and use that to log into the tablet.
I have done this for my kids, it means that they can use certain functions, such as hangouts, which they use to message and video call me when i am away. It also means that s they get older, they have a gmail account and if they want to purchase books, films etc, they are on their own account.
Knowing there account name and password, means i can set up all their accounts on my device, keep them seperate, but still monitor their usage.
Either way, works, it depend i think on how old your kids are, and how important keeping the usage seperate.
G