Setup. One goal and a penalty spot. For U12, bring the spot in a couple of metres.
How it runs. A round robin. Each keeper faces one penalty from every other keeper and takes one against every other keeper. A save is three points, a penalty that misses the target is one point to the keeper, and a goal is one point to the taker. Add it up at the end. The keeper must have one foot on the line until the ball is struck.
Make it harder. Sudden death between the top two. Then takers must use their weaker foot.
With 4 keepers. Everyone plays everyone: 12 penalties.