We bought Sophia a trail-a-bike so that we can go on some nice family bike rides as the weather improves. She isn't quite biking on her own yet (her bike is actually too small for her to properly find her balance) and not having grown up in a biking community, both Troy & I are still nervous about Sophia having her own bike on the road. Too many cars, peds, bikers that ride really aggressively. We'd much rather have her on the back of ours, and learn the rules of the road that way.
Alexandra had a great time in the seat, apart from the fact that her feet don't fit all the way in the seat stirrups, and she pushes against my back for most of the ride.