Well, going off ONLY what you have mentioned here (and your post makes me think there may be more going on than was written), I think it is unrealistic to think that if four children are already outside playing tag that a 3 and 5 year old aren't going to want to play too. I think that anytime kids are outside playing, it's unrealistic to think that other kids aren't going to want to play too. I actually think it's a little rude to say 'Sorry, even though my kids are playing tag and having fun outside, YOU can't play too'. That makes me sad, although I DO realize in certain instances (a planned BBQ or party they are not invited to) it is acceptable to not want to be infringed upon.
I must admit though, that this is a sensitive issue for me. A couple of days ago, my son's best friend got in trouble and wasn't allowed to play with my son for a few days (the boy's dad's consequence for his son, the boy hit my son with a bat, so they needed the break). The boy was outside the next day, though, and had started a neighborhood soccer game in the front yard. My son went over to watch (not play-he knew they weren't supposed to 'play' together) but the neighbor dad told him he couldn't even sit on the sidewalk (not on their property) and watch! So, instead of his own child being punished and kept in the house, MY son was punished by not being allowed to play with ANY of the other neighborhood kids and being sent home. So wrong.
That said, they don't need to be riding around your cars or taking toys out of the garage without asking and you need to let them know or just send them home.
I don't think the parents are taking advantage. After all, they ARE outside, just not up in your face. It's not like they are inside watching TV. Annoying? Maybe. Taking advantage? No.
And as for the lawn? Boundry issues? I don't think so, what point would he be making by purposely doing that? Maybe the actual property lines are blurred for him and he doesn't realize he is infringing on your property. If it bothers you, just nicely explain that you would appreciate it if he didn't mow those two swipes on your lawn.