I purchased a JP Vision 3.5 meter junior sail and while the sail looks like it is well made the boom front would have to be the weakest / cheapest piece of equipment I have come across in a long time, the picture shows both the JP and Ezzy boom front, the Ezzy boom is for sails up to 3m but it looks like it could be off any full size boom, the JP boom moves even when children are using it. Has anyone changed the front to a quality front and how hard was it.

Moves? Looks like old boom heads I've had that worked no problems. Is the rope tight enough?

Slip a bit of bicycle inner tube over the back of the clamp, the hinged bit, to stop it slipping.

My kids have the same JP 3.5 rig. Only used it a couple of times so far but no problems with the boom moving, even when I use it. I use plenty of tension on the clamp. I assume you are using the mast that came in the kit?