What would you do?

There is a park we like that is a very quick walk from our house. There are many houses who have a backyard that is almost the small park. But some are pretty far away, yet still are in the same grassy area. There is a little boy that keeps showing up. He is four. He lives at one of the houses described, but his is almost the furthest away. I've seen this little boy many times. Yet, we've seen his mom maybe twice. There are rules here on post about how old a kid can be to be unsupervised. Obviously, four years old is still a bit young (note the sarcasm) to be unsupervised. I've told him that his mom needs to be out there. His older seven year old sister informed us that her dad is deployed (which means nothing to me. whether or not her husband is deployed does not inhibit or take away from her responsibility to her child), and that their baby sister was sick. So, if you as a parent cannot go out, don't send out your kids. She could sit outside her house and listen and watch, which isn't ideal, but I can live with that. The other day, their back door was shut, and I watched the house for a while looking for movement of any kind in the windows. There was none. Today, she was standing at the back screen door one or twice, but that door is made mostly of glass and she wouldn't be able to hear anything unless she opened the door. Last time I saw them, I had told them that their mom needs to be out there. Today, that little boy came up to us (I was with a friend) and told us to mind our own business. Well, I don't think so. It is completely my business when her son is throwing sand at our children, and coming up to us to ask for snacks and drinks. The next time I go to the park, and that little boy comes out, I'm going to go to her back door and ask very nicely for her to come out and watch her four year old. The seven year old, as much as she still needs to be supervised, knows enough (I hope) to not go in the road (which is right next to this park) and to be more or less well behaved. But I've had to correct both of their behaviors at one time or another. I have a feeling that this woman might give me a problem when I ask her to come out. I have a notice that housing sent out with the rules about kids, and how old they can be to be left alone. So, I'm not making it up. This keeps happening to me when we go to parks on post. I'm tired of it. So, would you do the same? And what would you say? I don't want to allow it to get angry, and I want to stay calm (which can be really hard for me to do!). I'm all ears for your advice.......