I got some very good advice from Centre for Separated Families. They publish a book called "Putting children first" that you can get from them or Amazon.
I found this book an invaluable advice/tool to set up how our new separated family was going to operate. Including advice on how to tell DS. Try to read it first, agree between you and DH what you are going to do, and then, when you have the things clear (like knowing when one is moving out, which days they will see the other parent, etc), tell the children.
The book mentions that it is better to give the information in a more relaxed setting than sitting them very seriously to inform them is over.
With DS, we told him when he was playing in the bath (mind you, he was 4). He was a bit shocked at the beginning but we reassured him that he will continue to see his dad very often, told him about the "special trip" to his daddy's house every other weekend and about his ne bunkerbed... at that point he forgot he had a mother and became very entusiastic about it all It is almost 2 years now, and so far I can say he seems happily adapted.
I understand this aproach may/will not work for older children, but give the Centre a ring, I think I received excellent advise, and also felt well taken care of (they rang from time to time to check how we were doing, and at that point, that meant a lot to me )