We have a 4 year old daughter (43lbs) and we have been riding into work for the past 2 years.
The bike ride is not so long but as I said, we live in the city but we do use bike lanes (Toronto).
Our little one is now officially too big for her Co-Pilot so we are looking for something else for this summer. It seems that all on-board solutions are out of the question, but I have seen a product that seems interesting, its basically a seat that mounts onto the centre pole and has pegs. Anybody know what I'm talking about?
Our other option is to get a trail-a-bike, but we are a bit concerned about the logistics. How hard is it to switch bikes? Can I buy 2 sets of mounting hardware and swap the unit easily? How maneuverable are these trail-a-bikes? We do ride in the city.
TAB's work very well. Since it sounds like you ride with your daughter fairly frequently (I'm not sure if you ride her to school or just on weekends) I'd suggest the Burley. They make the multi speed Piccolo and the single speed Kazoo. I prefer the Piccolo because it gives them a chance to learn to switch gears especially since you will likely pull them faster than they can ride on their own. Not so important now, but when they are ~6 they will start to be able to shift.
You can buy extra racks for the Burley.
Adams and other seat post mounts are the other TAB options. They work ok and are not an issue when properly adjusted and maintained. They do seem to have more problems with setting up correctly and developing play in the system.
I can't think of any other options unless you go for a multiseat bike (tandem, recumbent, etc.).