My 3 now 4 year old is plenty small for our tandem trail-a-bike, the cranks are way to long, but she is happy.
We put toe clips on and strap the kids in... It works great!
My brother drilled and tapped new holes in the stoker crank arms to make his kid fit.
I tried to take off the pedals and mount pegs so she could ride along... but she wanted to pedal!!!!
I have seen a kid seat mounted on a trail-a-bike