I simply adore it when little kids acting like adults. My nieces and nephews do this all of the time. It is priceless when they put their hands on their hips and try to have a serious discussion with a grown up. Or when they pretend to go to work and do whatever their childs' mind thinks adults must do at work and they have their brow all furrowed.
Adorable. Every time.
On the other hand, adults acting like kids, this is a whole bunch of disturbing. The only exception being Tom Hanks in Big.
I guess you could call it a child's sense of wonder.
They're never cynical or apathetic. Everything is interesting, even the stupid things, and it just reminds you how beautiful everything can be if you just look at it like it's new.