In all honesty, it sounds to me like they aren't ready to be going over the table. If you have kids balking and so forth, that's pretty much a red flag that they are scared - which is normal if you haven't done the progressions properly. My suggestion would be to spend time vaulting over a stack of 8 inch mats or a resi/porta pit for a while. Get them comfortable and confident with that and then take the vault all the way down, cover it with mats, and put a resi in behind it. Start by having them just vault to handstand and fall to their back. Over time, uncover the vault and continue to progress until they are actually performing the vault.
Focus most of your coaching cues on the run, the hurdle, position on the board, maintaining a tight shape, and looking at their hands as they contact the table. If they aren't seeing their hands, then you can forget about blocking as the action of blocking. They'll "roll" over the vault.
So, you can keep scratching your head and looking for the magic drill. But, the reality is that you'll have far better success if you create a series of progressions and take your time at each phase to ensure that the kids are confident and ready to move forward.
A lot of folks use the words "drill" and "progression" synonymously. But, they are far different. A "drill" is a tool to fix a part of a pre-existing skill. A progression is a single drill/exercise amongst a series of drills that are derived from the desired skill. Hence, progressions are smaller parts of the new skill. These drills are then performed in a step-wise manner to enable the gymnast to learn a new skill.
The point is - you can do drills 24 hours, 7 days a week, 365 days a year...but, if they aren't being used in a logical, systematic format, then they are utterly useless.