Mine were 8 and 3 when we separated. The 8yo cried, the 3yo didn't know what was happening. That was back in 1999 so I don't even remember what we told them. We all survived and they are smart, well adjusted, loving kids.

Make sure they know that they are loved and that nothing will ever change that. It will be hard but it should all be ok eventually. My thoughts are with you.