Plenty of repetition and realism in training practices. Also be patient, they will get there. Small short patterns of play may also help. 

Work out what the difference is between training and match day.

Most likely it's numbers matched ( no overloads)

Pitch size 

Intensity or quality of oppo press

Pressure to not make a mistake 

I guess there are a few options here - one is the fear of making of a mistake. Are the kids or parents having a go at the kids if they concede a goal or miss a pass? That's one to cut out as if the kids are worried about it then they'll panic and ignore all the good work in training. 

Is it that training isn't the same level of intensity? in which case as has been said, overloads help here. 

Our team were a little like this and the one thing that helped the most was playing 5-a-side short games. We had three teams, three minute matches on a small pitch (a half of a third of an astronaut pitch) and we rotated the teams, not just winner stays on. We picked the teams to be even. This short fast passing focussed on control and movement and really brought up their confidence to attack quickly, but also to turn the ball over quickly too. It took a while (by no means was it overnight!) but now they're immensely confident with knocking it about. 

Hi Chris - thanks for sharing the above. 

Albeit challenging for players and coaches - my biggest advice would be to remain both consistent and patient with your messaging to players. One way to remain consistent is to use similar challenges/coaching points between both training and matches. There is also some great ideas already shared around some of the practical differences; intensity, players being outnumbered etc. 

- Can you continue/start using same coaching points from training?

- Can you encourage practices where there is an overload (regardless of topic)? 

- Can you try the above in two smaller identical set ups (parallel practices) so "building the attack" or whatever the possession-linked topic is can be achieve on a smaller scale? This will be easier than practicing with all players at first.

- Can you use conditions that avoid longer/lofted passes if that comes away from your session focus? 4 R's Repetition, Realism, Relevance, Reward. Big emphasis on avoiding the word "must" and instead offer conditions as an alternative option for players decision making to try when/where they feel best. Some examples below. 

SSG Conditions - Possession/Play Forward:
1. SSG restarts from either GK. If you play forward through each third without losing possession/out of play and score = 3 goals.

2. Every pass in build up = goal. So make 3 passes and score? 3 goals etc. 

3. A team cannot pass backwards twice in a row. Free-kick awarded from where second pass is made from.

4. Wall-pass and score? Worth 2 goals.

5. Every player beaten with a forward pass = how much a goal is worth (more around penetrative passing). 

Hope the above helps!
Lloyd