I saw a whole group of toddlers on an excursion tied together 15-20 years ago. It seemed liked the face of the future then, but I've never seen it happen again.
How many adults were supervising them?
It seems like a whole different world from when my generation were children in the late '60s/early '70s. The school I attended in Grade 1 was a county school, but located within the city. We had an outing one day, when we were allowed to go to the local theatre and see a movie (one of the Tarzan movies, as I recall). There were no kid leashes, and I don't recall more than two teachers supervising us. We were a class of 6-year-old kids, and we walked from the school to the theatre - in two lines, holding the hand of a "buddy" (not sure why anyone thought that was a good idea; boys at that age hate holding hands, and a lot of girls find it awkward as well). We got to the theatre, watched the movie, and got back to the school, and everything was fine. Nobody wandered off, nobody got into trouble, and it was fun.
The first time I heard the term "play date" was in the late '80s, on a soap opera. It didn't make any sense - the kids in question weren't even 3 years old, and certainly wouldn't have cared one way or the other. But of course this was a soap so the play date was just an excuse for the two single parents to get together.
I grew up in a time when kids just decided for themselves if they wanted to play together. Assuming they weren't grounded or had other obligations like chores, mealtime, or some family thing, kids would spend hours playing at someone's home, at the playground, or exploring the neighborhood. As long as homework, chores, and meals were done at the proper time, parental control was minimal.