I'm so, so sorry you are going through this. You sound like a woman in shellshock.
for you.
First things first: please find someone you can speak to about this. I know it might seem like money at a time when things are so uncertain, but the no 1 priority right now is you holding things together. It will really help to have a therapist you trust and can confide in over the next few months. You need to be this tower of strength for the children, and that won't be as possible unless you have someone supporting you.
Secondly, TELL PEOPLE! Your family, your friends. Do NOT make the mistake I did of feeling humiliated and ashamed and hiding it - this is not your fault, it does not reflect poorly on you in any way. Your partner will be spreading the word, and you need to build your team. Getting through a divorce is so much easier when you have people in your corner. (I did it alone because by the time I told people my ex had got around and accused me of cheating to the whole world - when actually he was the one who was unfaithful. Doing it in isolation is not something I would wish on anyone).
Thirdly, I realise he is saying and doing all the right things about money right now, but get thee to a lawyer, as fast as you can. I realise that it will seem like it's confirming your worst fears, but the sooner you know what your options really are, the sooner you can make plans for every eventuality. I have seen so many divorces start out with promises to take care of the kids, and end in acrimony, that I am the hugest fan of planning for the worst case scenario. It means you are forewarned and forearmed. This horrible news has caught you on the hop, so don't be on the back foot with him. He has managed to spring the worst surprise of your life on you - don't let him do that to you ever again.
Sending love, hugs, good wishes.