Hi everyone.I coach an under 12's rugby union squad.?

Hi everyone. I coach an under 12's rugby union squad. We have a great pack of forwards and some good backs. However, linking the 2 with the 9 and 10 are proving to be difficult. We cannot seem to get across to the number 10 about passing it rather than running it 99 times out of 100. Any suggestions or drills would be most gratefully recieved. Many thanks