No simple solution here. But one place to start is for the kids to gain an understanding of why they shouldn't bunch.  I do the following exercise to try and illustrate the point to the kids in a meaningful way.  

*Mark out a small area.  It depends on how many kids you have but I often find dividing one of the  thirds in half, or a quarter  does the trick.

*Kids are paired into one attacker and one defender and originally I act as thrower.  The attacker is tasked with getting free and receiving a pass in the area that you have marked off. Most kids have no trouble with this. As it is happening I talk to the kids highlighting the way they lost the defender.  eg nice lead, nice dodge etc... 

After all the team has had a number of turns we do the same thing except this time the other pairs stand in the area with their hands up. Now the players have trouble completing a straight drive or a dodge because players are in their way and other hands can grab the ball. 

I know it sounds basic but I find the kids get the idea really quickly.  With the younger ones I explain that the spaces where we need to catch the balls are stages and only one diva whould be moving into it at a time. 

It is also important for the kids to have an understanding of who should be going for the ball. Wiser heads than mine have helped me at the club I am coaching at and they are big believers in the team  bringing the ball from behind the defence goal line one throw at a time and looking at the options for each throw.  Who could go for the ball and who shouldn't and why.  

Good luck with this X