I know Ladios beat me to it, but here I go:
came over here and said do you want to be my friend?
The kid ask if he wanted to be your friend. By telling him no, you kinda started the whole thing. Yes, they probably were only saying that so they could use your trampoline, but either A. You would get to uses his/their stuff in return and the two-four of you would be best friends, B. They'd use it once and get bored of it, C. Continually use it, even without your permission. If they chose C, then yes, they were in the wrong. But you never gave them a chance for the other two, so you are in the wrong. You started the problem.