OP, I am so sorry to read this. A lot of women on here have been through this, myself included, and know exactly what you are going through/feeling. It will get better in time, but I know that you won't see that for a while. It was 13 years ago for me when XH walked out on me and then 4yo DD. He did the "I'm not happy, I don't love you any more" speech, while claiming that his new "friend" was just a friend. (He is married to her now). It was such a shock to me that I threw up. He had given no idea that he was unhappy, and life had been normal up to that point.
It's such a cliché, but things will get better over time, and it takes longer for some than others. Be kind to yourself, get support from friends and family. Eat little and often, get help with your DC. Get signed off work if necessary (if you are working). Get yourself some counselling to talk through everything. You do need to grieve the loss of him and the relationship.
Once they fall for someone else, you become the enemy and everything that is wrong in their life. You want to work on it, but he checked out a long time ago.
Don't beg him to come back. You need to realise that he is a different person now to the one that you think you know. They do turn into cold hard strangers. Everything will be your fault. They will spin it to others that he had no choice but to leave you. Meanwhile most people will think he is a piece of shit for leaving 2 small children.
He won't understand how you feel, he will want you to deal with it and move on. Meanwhile, you are grieving for the end of your marriage, and the end of life as you knew it, and also for the loss of the future that you thought you would have.
I am 13 years on now, my life is ok, I remain single by choice. He moved on and recreated our family with someone else and sees DD around once a year, his choice. He moved away, didn't call or see her regularly. Only time will tell how good a father your H is, but you do need to separate the 2 issues, husband/father. He can still be a good father if he wants to be.
You need to get all your financial info together, as others are saying, copy everything, pensions, life insurance, mortgage, savings, premium bonds. I know somebody who took all their savings, said he had no pension etc, so you need the evidence of whatever you have, to take to a solicitor.
Get online and see if you are entitled to any benefits, make sure the child benefit is in your name if you are entitled to it, and look at how much he should be paying you in maintenance.
I know that it all seems like a bit much, but the more prepared you are, the less scared you will be.
Keep posting on here for support