This pas year, I have used our Xtracycle with my oldest son (now 7, about 55 lbs) for getting to school with great success. In a pinch, and on flat roads I can take both kids (DS2 is 4 and just got the Hooptie which I quite like), but regardless it's time to upgrade my older son to a TAB.
I scored a Burley Piccolo on kijiji, which I'm really happy about and was planning to attach it to my main commuter bike, a Linus 3 speed. That's not crazy, right? Will I be nuts go up (not big) hills with just 3 speeds and the weight of my son on the back? I love the internal gears of the Linus, and 3 speed is all I've needed on my own, for the relatively small hills in our area.