The whole point of having you in cryostasis is that you could survive for a very long time and not age. All I'm saying is that if your child also survived into the vault, and if they were awakened from cryostasis before you were then they could potentially be the same age as you when you found them. They could even turn out to be the main antagonist without you even knowing that they were still alive, which would be an awesome twist.
With regards to having a blank slate, look all the way back to Morrowind. You were a prisoner, but you could still completely invent yourself in a million different ways. Lets say that in Fallout 4 you were in the military and you were married with a child, the same would still apply. You could still play your character out in any way you like because that's still only one aspect of who you are.
As for the voice acting, this could most certainly be implemented without "breaking" the feel of the game. In all of the previous games you are given a finite number of responses for any given situation. If these options are voiced, will it really be that different?
And as for the likeness to the protagonist, many agree that the facial features are basically identical.
Finally, why would somebody break into a vault and steal a kid? What if it were an android who is programmed to preserve life and is therefore following protocol.