The neighbor kids kicked their ball over my fence and I'm going to keep it.
Why would you just keep the ball? Have they asked for it back?
They did, but I kind of blew it off.
You shouldn't keep the ball that you know isn't yours. That's stealing. You should give the ball back to your neighbor.
They wouldn't kick the ball over my fence if they really cared about it.
They're kids and it was clearly an accident. They even came and asked for it back so they care about the ball. You really should give it back because it doesn't belong to you.