Oh, man. Words just don't work sometimes.
Think about his good qualities. Sounds like that won't be hard to do.
Doesn't matter what I think, but for what it's worth it doesn't sound like he bailed out on you or his kids. It does sound like an accident. That still sucks, but now it's time to think of those kids... they're going to need you to be there. That doesn't mean you have to "be a man" and bottle it all up inside -- it's okay to let them know you miss their dad too.
It'll help them to know that.