Well, my DCs got told in the car on the way to school by their father, then dumped on the curb. When I found out what he had done I had to go to the school and pick them up -- they were all in a state of shock. It was news to me too at that point that he was going to be leaving. You're doing far better than that, even though it's very difficult to go through with it and say the words; you're still there after the words are out, and that's what matters.
I had to go through explaining to them all that our house had to be sold and we are now in the process of waiting for someone to make an offer, pack, find somewhere else smaller and cheaper to live, and then move. They have all got through that in their own ways, although I lost a lot of sleep over how I would tell them. I have to answer the same questions over and over again from the younger ones, who need reassurance more than the older ones do it seems.
I felt terrible telling them, first about the divorce and especially because of the way they first learned about it, and then on top of that after an intervening year, about the house sale. I felt as if I was dismantling their world, and I questioned myself endlessly as to whether this was the only option we had had, whether things had really been so bad that it had to come to this, that the children should have had this nightmare visited upon them. I wrestled long and hard with my (Catholic) conscience. In the end, I felt that as an adult I had a duty to protect the DCs from the negatives that came with life with my exH, and that there were enough of them to fully justify the decision that I eventually made, not just for the sake of my own health and safety but of theirs too. We have been poorer but mush happier since parting ways, and DD1, who was estranged from her father for very good reasons, and lived at home like a hermit in her room, rejoined us physically, to everyone's happiness.
There's no good time to go through any of this, but I think as long as you are able to explain and answer and be available for the children, and provide security and routine and reassurance to the best of your ability, or to honestly say "I don't know the answer to that, but we'll get through it together, I'll take care of you no matter what, none of this is your fault" the children will feel they won't fall through the cracks.