First thing I'd do is have a chat with the kids and their parents to lay down some ground rules. Rule one being: they are not allowed to climb the fence. Rule two being: they have to ask for and get permission to enter your yard to retrieve their ball. If you aren't home at the time, they just have to wait. Follow the rules and they get their ball back - in time. If that's not good enough, they will have to improve their throwing accuracy or take the game somewhere else.
If they're half-way decent kids, once they know there are rules, they'll obey them most of the time. If they're problem kids, they'll completely ignore the rules, at which point your conscience should be clear in terms of escalating measures for dealing with the incursions.