Mine were a bit younger than yours when we split. We told them together, we’ll be started to speak but was obviously going to fuck it up and make it about how sad he was, so I took over. Kept it brief. I got away with the ‘we don’t love each other anymore’ line, but I think it’s different for you.
The thing to do is keep it factual and honest. Tell them you’ve separated - you’ll only be confirming what they know already.
Be honest. Tell them you don’t know how this will pan out.
Answer their questions honestly. If they ask if anyone else is involved say yes. You don’t need to embellish. It’s ok to tell them he was sexting I think, because that’s as much as you know factually. They’ll have their own opinions on this and need to make sense of it.
Don’t get angry. Don’t rant. Remember you are talking about someone they love very much who has hurt someone they love very much. The worst separations I have seen are those where all the energy is put into hurt and anger.
Hug them, let them go and find some space of their own or hold them tight. They have each other too and they are in it together. There will be some comfort for them there.
They may even be relieved to have some answers.
It’s one of the worse things I have ever had to do. But I found the words and got it done. You will too. 