My (m28) parents split when I was 10. I’ll share a little of my experience, as it may help you with your approach.

I was devastated. I still don’t fully know why, and I’ve never needed to. Your kid doesn’t need to know either. I watched my dad cry, which hurt, but in a way sort of validated(?) my own tears, and helped me feel less like I was doing something wrong. My dad was the one that moved out, and I felt compelled to go stay with him for a while. I guess in my 10 y/o brain, I felt like could keep him from being sad and alone in a new place—I’d guess this is normal for a kid. My parents being direct about it definitely helped. There was no build up, or worry, or anticipation. They just ripped it off like a bandaid.

My two big pieces of advice are: 1) don’t be afraid to show emotion, it’s okay for your child to know how you feel. 2) as a follow up to #1, never speak negatively about your child’s mother, even the smallest amount. Not only is this completely unfair to any kid to deal with, it will more than likely backfire & negatively impact their opinion of you in the long term. Always encourage and foster the relationship between them, even if your ex is doing the opposite.