We got a U+1, which is a sort of tagalong but with two wheels so much more stable. It has its own gears (which you can lock out so the kids don't fiddle). Being stable like a trike meant we could put the child on it without having to balance it, and strap their feet into the pedals for security using toestraps. My daughter loved riding on this and developed very strong legs! By about 6 she could push me along without me pedalling, which was a great sight, we'd overtake other cyclists with me extorting her to pedal faster, this tiny demon out of the saddle pedalling furiously while I sat up not moving my feet.
I don't know if they are still made. Never saw many around, they were a bit niche. Sold ours on, then I think it was sold on again. There was a sturdy platform between the two wheels, on which we kept a large wicker basket with a lid. Great for shopping.