That sounds like a huge distraction from your peace and quiet. Hope they become bored with the trampoline and find a new toy. I don't think a willow would make much difference.

if the trampoline is close to your fence suggest they move it further away in case the 'bounce off' and hurt themselves on the fence. [unless they have the netting around it.] I'd probably go with the willow screening as it is cheapish and can be removed when the trampoline is not in use. if they are like my kids the trampoline was used in fits and starts. heavy use for 1-2 weeks then nothing for a month and just when OH was going to put it away they'd play on it again.

This very issue is one of the reasons we moved house. Up to 8 teenagers bouncing on a trampoline that was sited right up against our garden fence. And a trampoline in every garden of the six that backed on to our garden (we had a corner plot), not one of them near the owners house...all of them right up against the fence. I sympathise. I would suggest Bamboo...it grows fast, doesn't block out light and is evergreen. But make sure its a clumper, not a spreader. People have no manners these days. No way would I ever do that to my neighbours.

If you complain and spoil their fun, you'll be branded the wicked ole witch for life.

Pleached trees are expensive and might well be at risk in the situation you describe - it only wants one of the kids to crash into your pleached trees to destroy the effect of all that training, which is not impossible if the trampoline is right up against the fence.