This seems such a difficult situation. I just don't know what I would do.
Am I right in thinking the trampoline is over at her house, but you can just see it?
You could say you'd noticed your daughter has bruises and 'guess her son maybe does too.' I think you could say that you think such young children need to be supervised on a trampoline and follow basic rules such as one at a time.
That way you are keeping it abstract. I think that is true anyway. Imagine what a nasty accident there could be with head bumping for example. This is serious so if she won't co-operate you could just say you would rather your daughter does not play. This link below for example says not before 6 yrs old.
www.rospa.com/leisure-safety/advice/trampoline/
safety.lovetoknow.com/household-safety-tips/tips-trampoline-safety
Here is a relevant quote:
Establishing Rules
Along with supervision, you should establish clear rules for jumpers. While the AAP advises against trampoline use, it does offer a few suggested rules for parents who opt to have a trampoline at home, the main one being allow only one child on the trampoline at a time. This is because most trampoline injuries occur when multiple children are on the trampoline at the same time.
Along with the one child at a time rule, parents should set other rules, including:
No somersaults, flips or other fancy moves
All balls and other objects off the trampolines
No roughhousing
No pets
Stay on top of the trampoline, not underneath
No eating or drinking while on the trampoline
Older kids must stay off the trampoline while younger kids jump.
No one under the influence of alcohol or drugs allowed on the trampoline.
While kids may not like these rules, they'll help prevent injuries, making the trampoline safer and more enjoyable for others.
Actually OP it seems it is essential, not optional to supervise this.
About the toys - why not keep special ones out if the way?
The little son sounds not nice at all, but he is only 4 and just not being looked after. You may just have to withdraw rather than let your daughter suffer.
Good luck.