Embrace the Old Lady within. (I love my inner Old Lady.)
"Hey, Kids,I'd appreciate it if you would go play at your own house. Our yard is private property and we don't want kids playing in the yard. No more playing in our yard guys. Thanks."
Then watch them until they leave.
I don't think you can be any more polite and direct. If you are having a difficult time managing your own space, it might be time to watch where the kids head home to, and then just talk with the parents. I'm sure they would understand that their children are not being considerate.
And yes, if you know the kids are playing on uneven turf and don't put a stop to it, you could be sued.
Another remedy would be to print up a short, neighborly letter explaining the situation and claiming that you will not be responsible for the safety and well-being of the children who persist in using your yard, despite your requests not to. Perhaps these parents will have a nice conversation with their children about private property and what that means. And this might protect you legally, if neighbors had been informed.
Fences make great neighbors.