I'll disagree with the above poster. If I had to do it again, I'd get the weehoo trailer. Nice low center of mass, can be used with very young kids, and they can pedal when they want to.
Both of my kids can ride bikes on their own, and both still also ride with trail a bikes. My kids can ride twenty miles or more on their own, but they can attached to my bike (we're going to attempt around 30 miles this weekend with the both of them on a tandem trail a bike).