My daughter was interested largely from watching races with me on TV. I told her that when she got good at riding a bicycle with no training wheels that we'd get her a motorcycle. She took to it very quickly and skipped the whole training wheels on a motorcycle phase.

She rides a Razor MX350. It's a 24V electric bike, quiet enough to ride pretty much anywhere, quick enough to provide a thrill, and cheap. I feel like it was a very good choice for a first bike. It even puts up with me goofin off on it in the cul de sac, great lil toy for a couple hundred bucks.

Her three (as of last week) year old brother also rides it but doesn't have the same internal motivation that she does. I'm afraid that I may not be patient enough with him. It's hard not to be impatient about things like looking forward and not just pinning it wide open. I think he may need to get a little older before I'm really comfortable with him on that thing.