Sidecar.

http://www.chariotcarriers.com/engli...idecarrier.php

My now 2-1/2 year old has been in one since ~6mos and loves it, as do I. It's not a rigid platform like a motorcycle sidecar, so the bike leans in corners as usual. Kid is on the non-traffic side and next to you, not looking at the rear hub and eating dust. Drawbacks: won't work with many road bikes (with tight front ends, Steamroller might work), sidecar only (ie: no stroller/sleigh/bivy sack conversion kits).

You will want gears and brakes.