"Go to sleep" is more to the point, with a child you could say "it´s time to go to bed".
"You will go to sleep " would be used with some kind complement (more elaborating): Now you will go to sleep, or I will put you to sleep.
You´re going to sleep. would be talking about plans made at the moment, for example. Or like the above expression, with some kind of elaborating about, say, where, how or when, etc.