I'm a pretty permissive parent...but if my kids dont listen to me at the playground we leave. I can see this being more difficult with a young toddler who might not really get "please dont go up the slide" but then a child of that age is probably mostly hovering around the bottom of the slide rather than actually going all the way up anyway, but if a child is that young you're probably going to "stalk her every move" anyway (or at least follow her around/stay close by)...id think? The kids that seem to want to go UP the slide around here seem to be more in the 5+ range.
I was at the play area at the mall recently. Almost everyone there was in the 2-5 age range. Nobody was older. Every single kid was trying to go up the slide. It was a real big thing. I remind my daughter to watch for kids going down the slide, but typically in that space I do not need to watch her like a hawk because she is cautious, doesn't do anything that could get her hurt, there aren't kids who are way bigger than her, and she can't get out of the play area without me seeing. The other place we have this discussion is in the church nursery where it is 4 and under only. There we just remind them to watch for kids going down the slide. The slide there is one of those tiny indoor ones.
If there had been bigger kids going down the slide, and there hadn't been 5 other toddlers trying to go up it, my daughter probably wouldn't have even been interested. I would keep more of an eye on her if those situations arose.