We started out on a trail-a-bike but they are way too long to be safe on shared roadways. What we have now is a Bike Friday tandem...which, since BF is literally three blocks from my house, is more or less a custom job that looks like nothing on their website (it is a little like this bike, but ours has front drop bars, rear handlebar extensions, and a massive crank range so we can actually pedal it uphill). The little wheels make for a much shorter wheelbase (though it is easier to oversteer than a larger-wheeled bike) and the whole thing is only about a foot longer than my 700cc Trek touring bike (which could, frankly, be longer; I have been riding it a lot this fall and the toe-over with the fenders on is kind of shitty. I could put a new fork on it but maybe the thing to do is sell it and buy something else.)

The Fridays have a really low standover height, which makes them extremely adjustable for kids and as kids grow.

Oh, I think tandeming for the first time with anyone who I'm not big enough to counterbalance, if they started to take us over, would be a long-pants-and-sleeves-and-no-traffic-at-first situation. But when you're going from a trail-a-bike to a tandem, it's MUCH easier.

ETA: Oh yes, I always captain. I love them but I've been riding in traffic for almost 30 years; they can try it once they're bigger. I think they'd have to be over 5 feet to captain even on a Friday, and Herself is still about six inches off that.