watching some adventure racing a couple of years ago, they attached the weaker team member to a stronger rider with a length of bungee cord, seemed to work quite well. Might depend how old the child was as to how successful that would be.

Depends how old the child is.  I made a device for towing my son around 7 stanes blue routes.  He can already ride down most stuff they have but hasnt got the strength to get up it so needs a hand.  I can pack it in my camelback when not in use and it takes 5 seconds to fit for the ups.

I got the idea off a guy on here and made it in about 30mins.  If your kid is confident enough to ride down stuff on their own and have good bike control i can post a picture up tonight.  Didnt cost much either

Basically its some LARGE cable ties on the head tube of the kids bike

Foam covering on the adults seat post

Bungee cord fed through about 1.5-2ft of electrical conduit and attached at one end.

Also feed a similar length of adequate thickness tow rope through conduit and have a loop at one end (Adjustable) and the other end make it so that at just before full extension of the bungee the tow rope kicks in.  Essentially you are using the bungee to take up the slack and stop any jerking but the tow rope is doing the pulling.

Hard to explain