Definitely start making the girls shout the "hit" count for each jump.
Example: 1-2-3-FOUR-5-SIX-7-EIGHT for a triple toe, with you counting the regular # and your girls shouting the bold
(you could make them count the whole thing while still shouting the bold as well if that will help them better)
Additionally, a key for timing and sharpness is to "Listen for the clean" (should be able to hear them slap their legs from cleaning tight after landing a jump). Start with a single T jump to practice, and tell your girls that you are listening for one unanimous clean. The girls will hear the popcorn effect of the slap if they are not on time, and realize that they are not together. It also helps clean up jumpers that I call "floaters"' a.k.a girls that jump lazy and just kind of float up and down in their jump instead of hitting sharp movements.
So in summary, a T jump would sound like this:
1-2-3-FOUR (*SLAP!*)