What should I do my neighbor is Kleptomaniac?

Well, i don't really have much of a problem but my little sisters do. they are 8 and 4 and they like to play on our backyard a lot with their dolls and stuffed animals. We have these neighbors who are one of those people who have tons of kids (I think they have 12, with a 13th one on the way). So anyway, their 9th or 10th kid is 5 years old. Anyway, i think he is a kleptomaniac because our sisters often run in the house crying saying that Kevin (the kid) stole their dolls. I went to their house once because I am acquaintances with their 2nd child, and I saw some of my sisters' toys in his room, along with stuff like loose change, my mother's missing earring, their mail (unopened), some mail addressed to us, some mail addressed to our other neighbors, car keys, school supplies, a 9th grade textbook (guessing it was one of his siblings), even weeds and gr clippings. I don't get it, because some of these things are in our house. He must be very sneaky if he can get away from his family, creep into our house unnoticed, and take some stuff. My sisters are very sad and they are running out of toys and my parents don't want to buy any more. Plus this kid has been stealing from me, too, some of my stuff has started disappearing as well, so what can I do? I've tried talking to his parents but they work all day and it's too late to talk to them when they get home. It's really bugging all of us.