A friend of mine designs playgrounds for kids, and is very good at it (his designs win 90+% of the competitive bids he enters).
He's also worked hard to put in place a playground safety inspector certification system over here.
One of his core ideas is that there are two types of safety: subjective safety and objective safety.
For example, kids quickly learn that falling from a height is painful, and experiences such as that teaches them how to evaluate their own safety when say climbing. This is the subjective safety.
However they're usually not going to be able to correctly evaluate the safety of climbing a playhouse where they might get their head stuck between two planks because the opening between the planks were just right for their heads to fit, but not their bodies. Or that the hood drawstring in their jacket can get stuck in small wedges and openings, especially dangerous near slides. Such issues go under the objective safety.
Now, his point is that you can make playgrounds which are objectively very safe, without making them any less exciting. Part of the excitement comes from allowing the kids to explore their subjective safety boundaries, but a lot comes from the design itself.
He often works with one of the major suppliers of playground equipment on their new designs, but a large part of his secret sauce is how he places that equipment on the playground. Many playground designers (which at least here are mostly landscape architects) seem to just put one piece there and another over there, without giving much thought to facilitating the flow of spontaneous play from one piece of equipment to the next.
Anyway, he could explain this a lot better than me. I just help him out with some presentations and such every now and then.