I've used a Burley D'Lite to pull kids up to age 4, depending on height, and a sit-on trailer-bike type trailer (mine is a Burley Kazoo) from age 5 to 8(currently). I am looking at tandems for the future.

I've used these to take the kids to and from school, distances of a couple of miles each way, when I don't need to transport more than one or two of my kids at a time. I stick to streets posted 25-35 mph. I drop off the trailer at the house before I ride the rest of the way to work, and pick up the trailer from home before I go tto the school at the end of the day. Sometimes we stop by the ice cream store on the way home. My son wears his backback on the Kazoo trailer; other days I use panniers for extra gear.

There's more discussion about trailers in the Recreational and Family forum.