Ok here's the update...
I managed to get away from work in time to pick thing 2 up from gym club today a bit early so I could just have a peek at what they were doing.
The were running up, squatting on the box and then a variety of jumps off. That bit was fine, what wasn't is that the mats were sideways after the box. Ok if the child lands properly but forward momentum and all that, if the plunged forward off their particular box, one group would fall into stacked benches, one into the table with the water cooler and the third, the wall.
So I went and told the head my concerns and brought him out to see. The instructor was unaware of this as we viewed from outside the window.
Also as the kids were jumping off the boxes the instructor should have been standing close and to the side to catch. He was at the other end of the hall.
The head is going to tackle it, I will now leave it to his diplomacy and need for safe practice. I think he will handle it in a way that doesn't dent the confidence of the instructor.