I feel like he's lost his right to be there to announce it to them if he's just not around, and like others have said, he is perfectly willing to lie through his teeth that he'll be back "soon" and see them "often".

Kids are smart - they might already feel where this is going. They deserve a kid-appropriate version of, "Yes, we're divorcing, no, it's not a single bit your fault, we will both ALWAYS love you, and you guys won't see your dad too much this year except for when he comes to visit a few times on X, Y, and Z dates."

When I was in therapy for separation (from my shitty ex of many years whose kid I helped raise, we had sole custody) my therapist told me that out of sight, out of mind can be better for kids instead of constant disappointment through empty promises or not being able to follow through on sincere intentions. I don't honestly remember how we told his daughter, I think I blocked it out because it's still too painful because my ex wouldn't let me have a peaceful relationship with his daughter without fucking my world up, but I'm pretty sure we did it individually. His daughter was very sad, I'm still pretty devastated by the whole thing years later but I've had to just trust the universe that that child is strong, she knows I love her, and my therapist told me that she's likely doing just fine in moving forward with her growing, expanding world without constantly being reminded that I'm around.

With your children I agree, a counselor could be extremely helpful but you'll benefit in trusting that it will be very, very sad but they can and will survive this, even and especially if the truth isn't sugarcoated any more than it needs to be. Best of luck to you, sounds like you're 1000% doing the right thing for your children and yourself. When this is all behind you in the rearview mirror I hope you feel like a new woman - I know I did.