Practicing on a rail-trail might help you both get used to the trail-a-bike without any hills or cars to make life difficult. As I recall, we put our son on the trail-a-bike when he was 3, but we stayed off the roads for a while. (I have the easy job in all this. I’m the one riding shotgun.)
The leap in responsbility really shows once they’re on their own bikes. I was amazed last summer at how much our son’s riding improved on his single bike after riding with his father on the tandem. Also how much more eager he was to ride. He would even suggest going for a ride with (gasp!) his mother.