Finally! A question I can answer with some experience to back it up!
Stick with the simple bike. No gears, no brakes, they just aren't helpful. And when your kid gets frustrated with not being able to figure out the gears, they stop pedalling. Oh and the weight difference between the two setups is incredible.
I bought our oldest son the fancy pants geared trail a bike and we both couldn't wait to go back to the simpler bike. Now his 10yrs younger little brother is my tandem partner.
TrMama is also clearly speaking from experience. I have seen knuckle joints that have 4-5 different tightening points and makes for a really wobbly ride as they always come loose. And be prepared to come up with sleeve shims if you are moving the trailing bike to different captain bikes....we just went ahead and got a bracket for each of the road and mountain bikes so DH and I can carry the load equally for longer outings.
Once your little one gets to about 60lbs, you are going to want that bugger on his own bike, and little bro can just slide into the hand me down.