I'm so sorry, love. Like so many men, he has decided no longer being the centre of your universe that now includes other calls on your time, attention and love is not for him. Utter selfishness personified, and a tedious cliché.
You need your team.
Are parents / any siblings about? Please share with yours, you'll need their support and I hope they'll be there for you and your children with open arms.
Does his mother know? She will of course side with her son, but remember she will be thinking of the grandchildren too. Facilitate his parents seeing them.
If you work, even if off on mat leave, ensure your line manager is aware of what you're going through. Staff welfare is part of their job.
And remember you and the children have RIGHTS.
Find your rage and your steel within the grief and the shock.
See a solicitor. You're married, so you co-own everything. Believe them, not anything he might try to make you believe, because he is not your friend.
See a financial advisor.
Find out about his duties re child maintenance etc.
Keep records of every penny you spend on the children.
Keep communications with him formal, in writing (emails not texts, for the paper trail) and filed.
Be prepared for him to start reframing and retelling the story - 'if you hadn't... I'd never have ... Now look what you made me do' bullshit.
Remain polite and businesslike - the 'crazy ex' lie is common and you don't want to give him any ammunition.