I‘m pretty sure it‘s from a region in the wotld were weather is very presictable, typically good, and people go out woth kids only in nice days, because there are so many.

Looks like rain in one of the videos and it'll suffer wear from vibrations as well.

It's a nice, cheap material for prototyping on, and then you can base the cut of a piece of metal or plastic on it for a more permanent solution.

OSB wood?

OSB is not very nice either. Plywood is definitely the choice here.

I think MDF will easily last until his kids don't fit in it though. I don't think he's going to use it in the rain.

Advantech OSB will outlast conventional plywood by a mile in the rain. I exposed it, flat (the worst way possible) as subfloor, to rains for 1+ year while building my house with only maybe 1mm of fiber degrading on the surface a few places. Many modern OSBs use hydrophobic glues impermeable to water.

Maybe but it's basically made of splinters. Not something you want in a thing you have to touch!

That's what I was thinking. Plywood is a horrible choice.

Marine ply would be my choice

If it were going to be exposed to the elements, I would pick a pine and waterseal it or a marine ply and waterseal it.

It would fail eventually anyway, but either of those two options, depending on price, would be good and cheap enough to last.