How old are you boys vaulters? If they're jumping between 7'-8' they might be better served sticking at 4 for a while, there's probably still room to improve from that run and help them build reps and proper form.
If you feel they need to move, try to put them in a situation where they feel as confident as possible. In my experience sliding down the pole almost always indicates a lack of confidence more-so than inability athletically. The same goes with crazy changes on top of the pole. As you seem to imply, I think this issues are probably more mental than anything else. I would do some 6 step grass drills running past the pole or maybe some drills with a plant box to get them confident with the run and pole drop, and start them on their poles they use from 4 at 6. This should give them a tremendous confidence and help them attack the takeoff more, which may fix the problem right there.
dj brings up a good point too, on top of what he said, maybe put down a cone at their optimum takeoff mark so both you and they can see where they're supposed to be. Using these two checkmarks can help ensure that they at least get to their takeoff point correctly which goes a long way towards having a great jump.
Hope this was helpful, please feel free to disregard everything I said if it sounds crazy or you get better advice from someone with more experience