The tricky thing about buying a bike (or anything else) for a kid is that there's a 99.9-percent certainty they'll outgrow it. The genius behind the Kilobike is that while they're between the ages of 6 and 12, the bike will grow with them.
The Kilobike, a concept by German firm Kilo Design, comes with swappable joints in the frame's top tube, which expand it out diagonally. When teamed with standard seat- and handlebar-height adjustments, these joints allow the bike to grow right along with a child's ever-lengthening limbs while ensuring they ride with proper ergonomics. Not to mention saving mom and dad a few dollars over the years.
It's one of those conceptual designs that seem so simple that they would have to have existed in the mainstream by now, right?
[Kilo Design via Yanko Design