The skull

I haven't seen this movie for decades, since I was a kid myself, but one scene stood out to me, when the kids and the old man are newly shipwrecked and the boy finds a "white object with holes in it" and the old man throws it away, horrified, and tells them to stay away from such objects.

Then several scenes and time passes, and the old man dies, leaving the children all alone. His body is left exposed to the elements, as they didn't know what to do.

Then, years later, as teenagers, the boy comes across the remains of the old man and he sees the same "white object with holes in it" that so terrified the old man... but this object IS the old man.

And the boy finally understands what a skull is. I found that so moving, tragic and morbid, as a child myself.