As soon as I saw the fish jump I found a kid and dropped him in the water and was able to get the fish even though it had stopped jumping, or most of the time it worked, I think the game probably gives you a minute or two to find a kid, just drop him where you saw the fish jump.

As far as the boulder goes I've seen two different posts about it, one said that a child would move it on their own at a certain point in the game, another said the boulder was never moved and they had finished the game so who knows.