Be clear about what you want to achieve.
- You want him to know that you know he has cheated
- You want him to know that you will not tolerate this
- You want a separation
There will be lots of other things you want, like a full confession, an explanation, more details, etc. but I wouldn't expect all that on the first discussion.
All you need to achieve at first is those 3. He may say he hasn't cheated because it wasn't physical, blah, blah, blah. But what you mean by cheating is inappropriate contact with another woman, secretly, deliberately hiding it and lying about it. That is enough. That is cheating.
It's important to show that you won't tolerate it by not accepting any excuses, denial or minimising. By not telling him exactly what you know, you keep the upper hand. Just keep repeating, 'I know what you have done, I want you leave'.
The separation is the most important. He should leave the house right away, that same night. If he refuses you should tell him that you need time and space away from him to think and if he won't respect your wishes, you will go straight for divorce.
If he wants to save the relationship, he will leave. If he still refuses to leave, then just go to your mum's for the night and leave him at home with the children.
Of course, he may just walk out anyway, he may say he wants to be with OW and that will be hard but, remember, you don't want to be with someone who would treat you like this anyway.