Well, not really :) But maybe it's a very important kid? Anyway, both тобі and вам are correct translations
One of my fellow Norwegian contributors suggested that this kid might be a prince or a princess :D
But you know, there can be even less fancier situations. For example, it may be possible to use it if you are a servant and it's your master's kid. Or there are just these very fancy elders who refer to everyone with formal pronouns (these are becoming really rare) :)
Yeah, made me smile as well :)
But you know, ви/вас does not necessarily mean polite you, it is also plural you. So technically it could be addressing a group of kids... But then you would add something like "Are you all eight yeas old?" (Вам усім вісім років?)
So yeah, duohumour B]
I do not know if there is so much meaning in the phrase, it is just "Dative + number of years." It is a set construction.