Whatever the kids are big enough to do by themselves, they’re allowed to do. If they want to climb to top of the tall slides, and go down, they can. If they need help to climb up, (because they’ve never done it before), they can’t. Not until they can do it by themselves.
More times than not, this has led to a more relaxed dad, a more centered dad, a more, "Hunh, seeing as how you can't climb the ladder to the small platform you could plunge off of, you'll be OK." sort of dad. Occasionally things do run in the opposite direction ala, "Who knew you could climb the 12 foot tall pyramid o' ropes mounted on a merry-go-round?" In the latter cases, risk amelioration is achieved by Casually Hanging Out close to the Area Of Highest Concern', (e.g. directly under the kid giggling on top of said rope pyramid), tactfully concealing my inner-qualms.
The system tends to get the kids to try things sooner, and builds their independence and confidence as they cruise along all on their own. It does however, occasionally lead to issues with other parents. Recently, a panicked father found me; informed me that my 3 y.o. was climbing to the top of the highest slide, up the hill rather than on the stairs; told me that I should be more careful; and also informed me that he had enticed my kid back down with the offer of a cookie. He then stood awaiting my thanks and validation of his Playground Hero status. My only response was to tell him that, in general, you should never ever offer someone else’s kid food from strangers. Completely baffled, the errant dad wandered back away as my kid, now loaded with half an illicit cookie, trundled right back up the hill.
Goofy adults aside, the Rule has led to other unintended, but awesome consequences. On a hike with a group of home-schooled kids, we crested a hill to find a wall from a long-ago torn-down structure; a 10-foot long log from a fallen tree laid on the ground beside it. Intrigued, the kids immediately asked me to move the log into a crack in the wall. They wanted to use it as a ramp. After I told them they could put the log wherever they liked, but they had to do it themselves, they tried, but couldn’t lift it. Nonplussed, they meandered off to indulge in other activities, or so it seemed. A few minutes later they returned accompanied by a band of 8 others kids. Together, they mounted a new attack, splitting up with five kids to either side of the offending leviathan. With their combined effort, the log was quickly lifted into position, A great ruckus ensued, and a tremendous time was had by the 2 to 7-year-old lifting crew as they explored their newly created playscape.