The playground in our park has a "for what age" sign on the plaything. So, the toddler slide is only for toddlers up to 6 and the other bigger slide is only for kids over 6 and under 12.
So, I've always thought that adults are not allowed on any of the slides. Basically, if your toddler can't go down the thing by himself, then he shouldn't be on it.
Now, we went to the County Fair and they had this HUMUNGOUS slide... like 20 feet high at least. You walk up all these rickety steps, get to the top and you put a towel on the top of the slide, sit on the towel, and slide all the way down. It was awesome. There are 3 lanes on the slide, so friends can slide down together. But the people at the fair won't let you ride 2 people together on a towel, not even if you're the parent holding a child. You can go side by side (parent on one lane and child on the lane next to it) and hold hands, but you can't slide together. And yes, I always thought this was because you don't want to get the little kid rolled over by the much bigger parent.
But this leg fracture thing, I never really did think of it, but yeah, it does make a load of sense!