One thing to check out if you are considering tag-a-longs is the FollowMe Tandem. It is a bit pricey at $400, but in my experience there is only a minor "wobble" that comes from the kid pedaling. The great thing about the FollowMe is that it attaches to the rear axle of your bike. That keeps the center of gravity low and avoids the more pronounced wobbling caused when the kid's bike is attached to the seat post as with most of the tag-a-long designs.

The FollowMe is a little hard to get in the U.S., since it is made by a German company, but there are a few options for ordering it online. Only other caveat is that it is a bit of a beast (prob about 5 lbs), but you do want your kid secured to you with a strong, well-manufactured piece of metal, which it certainly is. The design also allows you to mount another kid's seat on your rack if you need to.

I've used the FollowMe with my kid's 16" bike, and now that he's getting older, I'm personally planning to go the tandem route - but if you think there are benefits to the tag-a-long system for you, the FollowMe is definitely the best of the bunch.