Another possible way of dealing with this is a quick brief at the coin toss with the captains.
" okay fellas down to penalties I'll just warn you now so there is no confusion the penalty kick is over when the ball either: stops moving and is on the ground, is in the back of the net ( ie a goal has been scored), the keeper has it in his hands or it goes out of play. Clear?"
Then let the keepers know exactly the same so they don't fall for this. May seem pedant ice but as this shows if could save your bacon
" okay fellas down to penalties I'll just warn you now so there is no confusion the penalty kick is over when the ball either: stops moving and is on the ground, is in the back of the net ( ie a goal has been scored), the keeper has it in his hands or it goes out of play. Clear?"
Then let the keepers know exactly the same so they don't fall for this. May seem pedant ice but as this shows if could save your bacon