Hi. My older kids were 8 and 10yo.
I found this book enormously helpful. Helping Your Kids Cope with Divorce the Sandcastles Way by M. Gary Neuman and Patricia Romanowski. Ideally both parents would read it. That wasn't the case for us but it was still great for me.
I also read this book to them. Mom's House, Dad's House for Kids: Feeling at Home in One Home or Two by Isolina Ricci Ph.D. It was particularly helpful for my 10-11yo. He does not talk so had zero input but I could read it and use it as a jumping point to talk about other stuff. I did read it to the 8yo. I may have helped a little. He was younger of course but much angrier with me and the divorce being my fault so he wasn't really open to listening to me.
I read this one to the 10-11yo. Divorce Is Not the End of the World: Zoe's and Evan's Coping Guide for Kids by Zoe Stern, Evan Stern I think it was helpful for him. I left that with him to read again if he wanted - I'm not sure if he did. I read them out-loud because he has dyslexia and his reading was really weak at that point. A 'reader' could probably read on their own at that age. It was also just a way for me to be with him and talk with him because he absolutely did not say one word about any of it and so there was only so much one sided conversation I could manage.
ETA: I also didn't give them a reason. I just said that mom makes the best decisions that she can and it was grown up stuff and that we both loved them very much and it was not their fault. I did talk to the older boys about alcoholism a little bit before the divorce - when he left for rehab. I did not relate it to the divorce or say it was dad's fault or anything like that. I said something along the lines of mom's and dad's sometimes can't live together anymore and that grown ups make those decisions for reasons that have nothing to do with the kids. I added that mom's and kid's are different. Mom's never leave their kids, ever, there is no reason or nothing could happen that would cause that. While technically that is not true it was true in their case so I went with it. Their dad said some scary things to them. I'm not sure if intentionally did that or not but either way I felt I needed to assure them of some things. I wouldn't leave them. I loved them. I was making the best decisions I could. Their dad could take care of himself - just like all adults do - we don't have to worry about him. He had pretty much told them I didn't care about him, them, or the family and that I'd left him to starve. Plenty more I didn't know about I'm sure.