Got it. Recommended speed drills? Sprints, races?
Make them do more with less.
Like a three step run, hurdle, and punch into a rebound up to a stand on port-a-pit plus an 8 inch mat.... or something stacked about that high. Use this drill to emphasize strong and fast strides, and to push into the board during their hurdle and the posture (feet in front). This is a great time to work on their arm circles as well because they can drill it in the slower motion of the three step run.
Really, the 3 step run can be repeated 20-30 times and tire the kids less than 10 full length runs, but at same time they get up to 30 opportunities to learn and tune their hurdle technique and punch posture. Don't worry about it being boring because they seem to get a kick out of seeing how high they can go when you add another 8 incher to the stack.
You can make it really fun by adding two steps to their run and getting rid of the top 8 inch mat (once they all are landing nicely) so they can do front tucks onto the stack...... and then add to the stack. Emphasize the running speed, rhythm, hurdle technique (includes arms), and posture. Done well they should be able to add to the stack until they're head height, and higher.
Once the have the five step (stride) run tuned to perfection (well kinda) and can salto to a stand onto a head high stack, you add two more strides and have them do handsprings on the table. They probably do them better with seven steps than they did all season long with a full length run.... even if they had decent runs to begin with.... because it doesn't work to run fast and hurdle the wrong way.
Expect.......
The littles to have a bit of difficulty wrapping their minds around the fact they're 44 inches tall and are fronting to a stack 60 inches high.
That the two weeks you spend messing with this will give them time to forget how to vault, and that's going to make it easier to learn the new way you want them to do it.
A three step run started from a feet together, lean and fall forward into the first step (kinda hard to mess up a lean into a fall forward) will measure 16-18 feet from the back of the board. So start at 16 and if the chop up the last step have em back up to 17 feet.... and 18 feet if they keep chopping the 3rd step (yeah, you gotta count them at first).
Each additional two strides will require 9-10 feet of runway for the 5 step run, another 10-11 feet for the seven step run, and 11 feet for each pair beyond seven steps. Those are close approximations that will be adjusted for height, speed, strength, experience, and little things like fatigue, sad moods, happy moods, meet adrenaline, and anything else. They'll stay consistent if they put their best effort into running fast (that's why you start at 3 steps so they get used to going "all out"), and the variation per mood and energy level can account for an inch plus or minus per stride.
Happy moods and fresh bodies + plus 1 inch per stride, and sad moods and fatigue - 1 inch per stride, at least for beginner and intermediate (handsprings) vaulters. The more advanced kids will have less variation, but it's still there, so keep an eye out for that last step chop or stretch and address it if it happens two times in a row or seems frequent.
Have fun.