WWYD: neighbor kids playing in my yard

[petesgirl]

We live on a corner lot near our neighborhood playground and a walking path. The walking path connects our community to a larger park and also to an apartment complex. There are a lot of kids that live in the apartments that wander through our neighborhood unsupervised. I haven't seen them cause much trouble however some of our neighbors have had to call the police on some of them before due to the kids throwing rocks and damaging property. Last night DS was in our backyard jumping on the trampoline and 2 of the kids saw him from the path and asked if they could jump so he invited them in. I don't have a problem with this. A few more kids straggled in until their were 5 on trampoline which I think is too many. It was getting late anyway and I told them it was time to head home. A few of them protested a bit and asked to stay longer but eventually they all left.
They are back today. They knocked but I didn't answer the door because I was on the phone and they just went around to the back yard (ungated on one side) and started jumping. I went out and told them that DS isn't home but they are welcome to play, but that they DO need to knock and only play if someone is home instead of just going to the backyard themselves. Now there are 6 kids in my back yard that I don't know.
I'm worried about them jumping on our trampoline and getting hurt. The few neighbor kids that come over to play with DS know we have a trampoline and their parents have told me it's ok for them to jump. I don't know the parents of these kids. I also don't like that they helped themselves to my back yard after I didn't answer my door and that more kids have joined them. I know they need an outside energy outlet that they don't get from apartment living and I want to be friendly and kind but.... WWYD? I'm afraid we are going to have all sorts of kids we don't know playing in our backyard when we aren't home and I'm not ok with that.