Lets start by not dismissing felony arson as an offense because the offender is 13, and acting like it's the designers' fault.

What are you gonna build a playground out of? Want to go back to wood playgrounds? Last I checked wood burns too.
Metal? Nevermind the obvious hazards associated with falling during that oddly cold season that seems to dominate 7 months out of the year and makes metal slippery, it could also be hot in the summer. And lets not forget, playgrounds tend to me places of play, where children are running around, playing games like "tag" etc, and generally managing to fall face/hands first into any number of objects that can hurt. At least plastic is a little more pleasant.

I say keep building plastic playgrounds, and teach punk kids that lighting fires is wrong. Seems to make everyone happy.