I'm upset at our neighbors for letting their kids be outside all day.
Why does it bother you if their kids are outside?
They are keeping their children in an unhealthy environment. They could get sick.
While it's good to worry about the welfare of a child, it doesn't sound like your neighbors are doing anything wrong. Kids love to be outside and it's good for them. You shouldn't be so judgmental of how others raise their children since people will view you negatively for doing that.
But what if they catch a cold?
I don't think it's likely that the kids will catch a cold from playing outside. I think it's actually safer for them outside. I think you should really let your neighbors raise their kids how they see fit.