Problem with new trailer, or is it the grandson?

Sounds like a curious kid growing up, being outside, climbing, hopefully he is jumping in mud puddles and skinning his knees. Just keep an eye on the kids, try to let them make their own mistakes (hopefully without being too dangerous - like on top of a camper), and don't jump all over them when they do a perfectly normal "kid thing", but explain how it is dangerous.

If the kid returns and still tries to do something like climb that ladder, then discipline him.