Sorry to hear you're splitting up, Lover, but it sounds like you and your H are being very sensible about it.
As a child of divorced parents myself (twice each...) I remember appreciating my Mum's honesty about the situation. Yes, it was painful and as a parent your instinct is to try and protect your DCs from hurtful truths, that's only natural.
But children can usually tell if something's being hidden from them and if you make sure they are in the picture, it will reinforce the trust they have in you and their father.
Snorbs is right - make it clear it's not their fault and you and Dad both love them as much as ever - and give them time (and space, if they need it) to absorb the news before your H moves out. If it's clear it's not a taboo subject, then hopefully that will encourage them to ask questions and share fears they have about it.
As I said at the top, I admire the way you and H are going about this. In the difficult days ahead, one thing I would say is continue to keep in mind that your roles as your DCs' parents are to some extent separate (now) from your personal relationship with each other, so no matter how hard things get between you and H, don't succumb to the temptation to drag your DCs into it. Even now, years later, I get upset if one of my parents badmouths the other - after all, however they treated each other, they are still Mum and Dad to me.
Hope this helps you! All the best, it's always a horrendous time, but I think you're already approaching it in a healthy way.