Well, just yesterday when I ate 3 entire slabs of chocolate during the course of a single day.
Though this question is kind of hard, because what is being a child? Was I, in the above example, being a gleefully indulgent kid or an unhealthy, irresponsible adult? I don't really see watching Avatar: TLA as liking a cartoon, I see it as liking a great television show. The line between what's child or adult can be blurry depending on your perception of things.
Though the most pure example would be watching this just a couple of days ago:
Suddenly I was six years old again, and remembered how funny bodily noises were as a kid, and the louder and longer they were, the funnier they became. I'm kind of glad that I didn't see that clip as a kid, because I seriously think I would have at least pissed myself laughing. If that skit doesn't bring out your inner six-year old, you simply don't have a soul, you wretched husk of a human being.