I'm looking at getting my kids into long rides and was thinking tandem, but what if I built a high performance tag-a-long? Most of what's available is for easy riding, not to transform a regular bike into a nice tandem. Also, my kids are getting a bit bigger, so looking at a 26/700c type wheel (they're 60-70 lbs), so it could grow with them for at least a few years before they can handle some distance on their own.

So has anybody done this or have any ideas?