Are the kids sticking to public areas, or are they actually going onto other people's property? Because if they're sticking to public areas, I don't see that the horses would be an issue at all. And at the 6-10 age range, they're old enough that they should be able to be somewhat independent and explore the neighborhood, etc. If there are specific dangers that you think the mom may not know about, due to being new to the neighborhood (snakes and how to watch out for them, dangerous dogs and where they might be, locations of any sex offenders that seem like they might be a particular danger to children (in so many cases, somebody seems to be on the child offender list because they were under 18 and had relations with somebody too much younger than them, but both were in high school, or what-not). Not so much as a, "So, you shouldn't let your children roam," thing, as much as a, "I noticed that your children are out and about a lot, and I wanted to make sure you knew what to warn them about."