punching of the board is something you wont be able to change a great deal, over a short period of time. Simple because its a matter of the gymnast explosivenes. You can see this simply by looking a group of kids. so will just be better able to punch of it then others.
SO! aside from teaching the kids to actually use their legs don't expect great! changes.
So what i recommend is LOTS and LOTS jumps on the trampoline working on all kids from jumping and landing with bent leg to working just the ankles, working jumps just with the arms. Lots and lots to develop the tension, leg shape, and ability to learn what it feel to stretch completly from them.
Second work the same on the actual board. Hold their hands and have them doing sets of 20 or so jumps in a row. With GOOD form.
Work on them having tight ankles, fast rebounds (keep encouraging it, even though if they say they are trying they hardest they probably are). If they have weak ankles they will not be able to impact the board without their heels hitting the board which means more time spent of the board.
Jumps on floor are also good, however remember to be carefull with not overdoing it, cause the consequence of overdoing it without proper conditioning of the ankles (flexibility and strength) will result in child Osteochondritis conditions to the ankles (eg. severse desiease) and knees (eg. Oschood-Schlatters Disease).
Conditioning like calf raises, theraband work, injury prevention specific exercise etc..
Also one other thing is check that you dont have to many springs in the board if the kids are really light.. i mean for really small kids on a regualar board about 2 spring on the left and right are with at the real bottom is about enough.
The Gym Press