OP, I feel so sorry for you.
I feel I'm missing something though (and apologies, I HAVE RTFT, but did so on my phone and may have missed a post.)
You had a discussion with your DH, he admitted to chasing her, sending flowers, the stuff about the song lyrics, but insists that they never met up. He refused to give you access to FB, WhatsApp or toher corrspidnecne because he felt you would erad things into it (or some other patronising guff).
Then you say he is leaving tonight and you are both telling the kids tonight.
Is there something that happened in between these two things? Has he admitted to more? Are you kicking him out (on the basis of the admitted indiscretions, or because you believe there is more to it?) Or is he going of his own volition?
I think in terms of the kids, you should come up with a 'holding story' to tell them from now, to buy you some time to decide what ultimately to tell them, if indeed you do decide to split. I certainly wouldn't be doing the 'daddy has been a cunt and doesn't love us anymore' script right now. Too premature my lovely. Also, you do need to find out more- you need to get your husband to talk to you truthfully. It's the only possible way back for your marriage (if indeed salvage is even still on your agenda. Nobody could blame you if this was already too much.)
x