Goalkeepers are not permitted to punt or dropkick the ball.
No intentional heading the ball.
Use of the Build Out Line is required for goal kicks and when the goalkeeper controls the ball.
Build Out Line (BOL) video
The keeper can wait until the opposing team is beyond the BOL before putting the ball in play, the six seconds do not start until the opponents are beyond the line.
Opponents are to make every effort to retreat beyond the build out line when the keeper gains control of the ball and for goal kicks.
If the keeper puts the ball in play before the opposing team has had a chance to get behind the BOL, the opposing team is not allowed to make a move to intercept the ball. However, if the ball is sent directly to them, they are allowed to take it and play from there.
Note the interpretation that the opposing team -- a player who has not gone behind the BOL -- cannot play the ball unless it goes directly to them. Taking even a step to the left or a step to the right to get the ball means it didn't go directly to them.
Best thing to do is just tell your kids that as soon as the other team's keeper makes a save, or the ball goes out for a goal kick, just hustle back behind the BOL. Then you'll be in position to aggressively run in as soon as the keeper releases the ball.
Remember, the Build Out Line is a training initiative for building out play from the back. When a decision by the referee needs to be made, it should error on the side of the goal keeper's team.