How to tell our kid and how much info?

Hi! Last April, I (45F) found out that my partner (52M) was cheating on me since 2020 with the same woman and they have two young kids together. Despite the fact he wanted us to reconcile, I decided to separate (we are not married). We have a 9 years old daughter. How do we annonce her that we are separating? Do we talk about her half-siblings and the infidelity? What did you do?