I'll usually become quite childish when there are children around that I know. However, to me it isn't acting as I just seem to very easily return into childhood, and I find that their sense of amazement at the world is quite envigorating. Two examples I can think of:
1. Any time my Nephew is around, we'll start wrestling (or playing ding-ding, as he calls it

) or doing whatever it is that he's into at the moment.
2. My girlfriend has a cousin who's about ten years old, but she's very energetic and she likes to laugh and run around, and that's basically what we'll do whenever we see each other.
Granted, when I'm quite tired, I may not be as disposed towards doing those things, but in general I'm lively enough to have a little fun with them, and I don't have to force myself to do it.
Jam it back in, in the dark.