You should take a look at Bike Friday, they are small and very child friendly, plus I think they have very good pricing and if you find something used better. I think they are easier to ride than big wheel tandem since the wheelbase is shorter. I have a Twosday and I am getting ready to fit it for my 4 years old. They have the independent pedaling system and crank shorteners. Sometime on their website they have used bikes too. Also very easy to tranport since they fold and fit in 2 suitcases.
Good luck
Ciao
Paolo