I'd take the machine and hold on to it for a while before using it. My childhood was pretty damn good, with barely any bad stuff, so I certainly wouldn't mind giving it a whirl again, maybe making a few changes. That said, once I hit my teenage years, there are some things that I should have done differently, but I'm not entirely sure HOW. But I would do it, probably within the year. Growing up again would give me a good environment to improve my social skills, for one - take risks, see if they pay off, etc. And of course, if I get to keep some foreknowledge, I'd warn a few people to get cancer screenings early. Maybe make a few investments.
But for the most part, I wouldn't change things. I'd still read and read like there was no tomorrow. I'd still want to be friends with my best friend, because he's my best friend for a reason. I'd still love my very extended family. I'd still play dumb about my parent's marital problems. And I'd keep the same attitude, more or less. Maybe I'd do some acting earlier, who knows. Point is, I'd love it.
So yeah, sign me up.