Is there a such thing as a ground rule in the park Hr??

Here's the situation..

12 year old baseball and we were playing in a field that was bigger than regulation (280 down the lines and 300 to center). Normal for that age group would be 220ish down the lines. That's a lot of extra ground for a 12 year old to cover and a ball in the gap is a guaranteed triple or in the park home run.

Ball is hit to left field, past the left fielder. There is a crack under the fence and the ball clearly roles under the hole. I could see it easily from the first base dug out, so I'm saying it is out of play. The runner has already rounded second base and is on his way to third as it roles under the fence. Left fielder never threw up his hands, he just stopped when the ball disappeared. The kid keeps running and scores. Umps say since the kid never threw up his hands, and the base runner was already past second the plays stays as is and is awarded an in the park home run.

Does the rule say that a kid must throw his hands up to stop play? I have seen plenty of triples taken away because the ball bounces over the fence. How is this any different?