It’s a great project overall but MDF is a very poor choice for the base. It will get destroyed as soon as it gets wet. At minimum they should have used exterior grade plywood, and painted it
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.
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.
It would fail eventually anyway, but either of those two options, depending on price, would be good and cheap enough to last.