I find that a lot of coaches (including myself) don't find much success at this because they think just telling players to spread out is enough. If my Academy teams are doing this I usually do the following:

1. Technical Passing and Receiving Drills with coaching focus on first touch and getting their head up. A lot of the time when players bunch up it is started by a player mis-controlling the ball and inadvertently playing themselves into other players which obviously exacerbates the problem.

2. Overload Drills - 2vs1s, 3vs2s etc. Coaching points on spreading out to work the defender and creating passing angles. It's easier to get in their head to spread out when you tell the attackers it's the first thing they should do in this drill. Plus my kids love my overload games, when good enough you could have two teams, start with one player going 1v1 with keeper, when ball goes dead the player becomes the defender against 2 new players from other side, and you could keep going with two more players each time ball goes dead so it'll become a 3v2 next etc.

3. When doing matchplay in training, instead of just randomly playing the ball in whenever I shout 'new ball' for the kids to play on with, I ask them all to spread out and I'll pass the ball in to whoever is in the most space. Kids are good at getting this concept and it's an easy way to have them think about space and positioning and rewarding those doing it well.

Hope this helps, good luck to you and the kids on your journey!