Sorry to hear about your troubles chocchip 
Mine were 4 months & 4 years when we split, so a bit younger. I did keep some things in mind when I told DS1 that might help you though.
Make it seem like a mutual decision, even when its not, they are too young to understand the complexities of who did what wrong & it will prevent them demonising one parent whilst the other is the 'good' parent. They will have questions but answer it age appropriately, so they don't need to know about an affair, they just need to know that mummy & daddy will be happier living apart. They will need to know that its not their fault though, and its nothing that they have done.
Focus on what will change for them, how they will have 2 bedrooms & 2 houses & that they can bring their toys between them both. Also tell them about the things that will stay the same, so their school will be the same etc. Present it as a positive for them.
(Its sounds so easy writing it down but ime its not easy to do when you are seething with bitterness & rage that his behaviour caused this. I've always told DS1 that I will tell him what happened when he's older & I will, as he knows I'm keeping stuff back but how do you explain alcoholism & drug addiction to a 10 year old. I found that 'I'll tell you when you're older' is a useful middle ground for me, as the child knows that they will get to hear the truth so they don't feel lied to but the issue is way too complex to explain to him now. You may find it useful, you may not..)
Last thing, give them time & loosen the reins a bit, some children may revert to younger behaviour for a time or they may get angry & lash out at you, its just a case of feeling your way carefully through the days once you have told them & make sure that you remain a constant loving presence & that they can ask you anything they like & you will do your best to answer it.