Find an adjustable stoker stem where the male adjustable part measures 28.6mm, remove the male adjustable part and put it aside. Buy a cheap, long, 28.6 seatpost and cut the seatpost clamp off the top. Put the bottom of the seatpost into the female part of the adjustable stoker stem. Clamp a standard 1-1/8" (28.6mm) stem to the top of the cut-off seatpost, and add your child stoker bars. That's it.
For a quick change option, put adult stoker bars on the original male part of the stoker stem, then you can swap back and forth from the kid bars to the adult bars.
PS- you want the bars above the seat for a kid, maybe 3-4".