This is Only you can save Mankind, by Terry Pratchett, published 1992.
Johnny Maxwell buys a new computer game called "Only you can save mankind", except when he meets the ScreeWee they refuse to fight and want to talk to him.
There was the ship again, right in the middle of his sights.
He picked up the joystick thoughtfully.
There was an immediate message on the screen. Well, not exactly a
message. More a picture. Half a dozen little egg-shaped blobs, with
tails. They didn't move.
What kind of message is that? he thought.
Perhaps there was a special message he ought to send. "Die, Creep"
didn't seem to fit properly at the moment.
He typed: Whats hpaening?
Immediately a reply appeared on the screen, in yellow letters.
We surrender. Do not shoot. See, we show you pictures of our children.
All the alien ships disappear from his screen, and when he goes back to the shop to complain, he finds other players there complaining about the same thing. Later he gets his friends to join him in the game, to bring supplies to the ScreeWee.