OP here. Thanks everyone for the responses. First off, I have to say that I'm relieved to know DH and I aren't the only ones who find this situation to be ridiculous. To be honest, I have seriously considered telling those two boys that they won't get their stuff back unless the music is turned down and they keep their trash out of our yard. It's retaliatory, I know. It's just the icing on the cake that we have to be disturbed in the evenings by two boys looking for their lost toys when we've spent the previous five months disturbed by their loud music.
For those who asked about the music, fencing, etc.:
We do have noise ordinances here, and we've called the police when these people are in violation. Noise violations aren't a high priority, so they may or may not get attention. In several instances, the police have told them to turn the music down, and they have complied. However, they comply only for the moment and then they crank it back up again. It's useless to call the police. We've been told that our only recourse is to sue them in civil court.
About the fencing: At present, there are two fences dividing our property. They have a 4-foot picket fence on their property, which serves only to keep people out of their yard. It doesn't do anything for privacy. We have a partial 6-foot privacy fence in our yard, which we can only assume the previous owners put up to block the view of these people in their pool. (It's an above-ground pool.) Also, our houses are situated on a downhill slope, and their property is higher than ours. This means that they may be able to see into our yard regardless of the height of any fence we build. City codes allow fences up to 8 feet in height. Even with an 8-foot fence, given the slope of the yards, the way our houses are situated, and the location of their speakers (mounted high above their sliding glass door) I'm not confident that we'd be able to screen out sound. I've checked into soundproofing materials, and I just don't know if it would work. The speaker that is aimed at our house is probably 15 feet above the ground from our backyard.
We probably will put up an 8 foot fence just on that side of our property because we can. It's really the only option we have to screen out the view of them and their pool. It might be worth trying sound insulation on the fence. That might help us when we're on the deck. Not sure it would help overall though. We also have talked about planting bamboo along the higher fence for additional screening.
Or we could move. And we really don't want to move. We've only been here two years, and we love our house and the neighborhood overall.