OK, thank you for the advice. I think that having fun or not for a kid depends on several things, such as the activity proposed for them and the kids' temperament. I remember I had party with my family when I was a child, and often there were one or two friends of mine (same age), and we enjoyed a lot together waiting midnight. I actually have very good memories. But I'm not sure how old I was, my first memories of New Year parties come when I was in primary school, so I suppose 6-7 years old.
I also have friends that keep their kids 2-3 y awake till midnight, but they are not BW mums and often keep their kids very late in the evening...
So, I kind of agree with you that my DS is probably too young for all this, and he'll probably not understand what is going on yet and, yes, I am scared of OT that could come the days after...