It depends on how high functioning the child is and how old he or she is too. A child under the age of six isn’t going to understand it one way or the other; he or she is just too young. Over the age of six, you still can’t really explain it to a child who’s on the lower end of the functioning spectrum. What you can do in that case is just let him or her know that both mommy and daddy love him or her very much but they won’t live in the same house anymore.
As for a child over the age of six who’s higher functioning, the simplest explanation is the best. Mommy and Daddy can’t live together anymore because they bug each other. They may not love each other anymore either, but they do love their children and want what is best for them. Daddy or Mommy will live somewhere else, in an apartment or another house. The children may visit as often as they want.
It’s also important that any child, with or without autism, be made aware that it’s nothing they did or didn’t do. It isn’t their fault and they can’t fix Mommy and Daddy’s problems. It’s something Mommy and Daddy need to work on alone. If the child is verbal, he or she may ask questions or express distress. Parents need to answer openly and honestly, but on a level the child(ren) can understand. They also need to provide comfort and loving support; actions have impact on the words already spoken.
As the child with autism grows, he or she may have more questions about relationships, divorce, and why Mommy and Daddy don’t love each other anymore. This is natural, and if the child is ready to ask questions, answers that are neutral and not negative should be given. Parents who badmouth each other when responding to their children’s questions make it that much less likely that children will come to them later.
Non-verbal children just need the basics. They will not ask further questions, obviously, but parents can still talk to him or her as though the child does hear and understand what is being said. It has been shown that many autistic adults understood things clearer than people gave them credit for when they were younger.