Plasma Car!!!
Ours is now going on 8 years of near daily use. Neighborhood favorite. In fact... a lot of kids didn't even get bikes until age 8+... and even those that have them often prefer "The Scoocher" (aka neighborhood nickname for the PlasmaCar). Super sturdy, they even take significant adult (250lbs) weight, although they're not super maneuverable at that weight, are built tough, super sturdy (we've only had 3 or 4 spills in 8 years with over 40 neighborhood kids, and that's with kids TRYING to dump them oer), agile... overall BEST ride on toy EVER.
We started requiring helmets at age 3 when the kids started doing hills. Never really 'needed' them, per se, but they can get going pretty fast!!!
If you have tile indoors... replace the wheels with rollerblade wheels (for the quiet).
http://www.amazon.com/s/?ie=UTF8&keywords=plasmacar&a...