That’s great, except that the “okay?” makes it sound like a request, and therefore optional, when it is in fact a non-negotiable directive. And once you’ve made something sound optional, what can you reasonably say when the person you’re speaking to declines the option? The kids I’ve known over the years, they’d split about 50/50 complying with that request and saying, “Nah, he can wait” and continuing to climb. You’ve not really left yourself any route for dealing with the second 50%, ya know? There’s absolutely no reason you can’t firmly but politely say to a kid, “No, someone is already trying to come down. You need to come down from there and wait over to the side where you won’t get kicked.” Real directives, ime, get more consistent good results than semi-requests.
Telling a kid to knock it off and stop breaking clearly posted rules that he’s old enough to read is harsher than I’d go right off the bat, but also not something I’d get my shorts in a wad over someone else doing. I mean, he’s old enough to read the rules, and he’s almost surely not allowed to do that on the school playground for safety reasons. It’s not like the idea that he shouldn’t be doing that is some bizarre foreign concept he’s never encountered before. Hell, knowing you’re not supposed to be climbing the slides is half the fun of climbing them.
As for the inherent safety or danger of what the kid was doing…yeah, most kids climb slides at some point, and most of the time it’s fine. I did it myself when alone or in a small group plenty of times without incident. We jumped out of moving swings without incident, too. But other kids I went to school with chipped teeth or busted lips or broke bones when they lost their grip on the slide they were climbing or timed their landing wrong. In one spectacularly horrifying incident, a fifth-grader scalped himself when he was climbing an enclosed slide and his foot slipped right as his head cleared the metal hood. His parents sued the school district, on the grounds that he should have been supervised closely enough to prevent him doing something so dangerous.
So the OP’s response wasn’t ideal, but it also wasn’t wholly unreasonable.