Hard as it will be, say nothing to him. Go grey rock. He gets a calm, bland, factual response to anything he asks. Ranting at him will give him the chance to argue his point back. Retreat. Kill him with fake kindness. It’ll drive him nuts.
Tell him YOU need time to sort your head out and work through YOUR feelings, so you’d appreciate him taking the children out, so YOU can do this. Be calm, let him think he’s doing you a favour and being kind (he isn’t I know!) This buys you time to get your ducks in a row without him pecking your head. Stress you would never keep the children from him, why on earth would he think that? Etc. But this is best for them as they won’t see you both upset. Stress everything as what’s best for the children - as it should be of course. This is a good counter to anything ridiculous he thinks he’s entitled to do.
Contact his parents, pleasantly. Tell them you are having issues, he’s decided he’s moving out and needs to see the children at their house for the time being. Which is bubbly legal in the circumstances. Nothing more. They’ll work out the rest by his reaction when they question him.
Set up a personal bank account NOW and get next months salary going in there. Transfer back to joint for necessary bills. Take your half out of the savings before he clears the lot. If questioned, it’s just for now while you get your head clear.
Get legal advice ASAP both about divorce/finances and arrangements for the children. Ask around for a recommendation. Both of you have marital rights to the home, however establishing who is primary carer for DC and therefore who should occupy it with them will take care of this. Any time he wants access, and won’t be put off, grit your teeth, get a clear timeframe and remove yourself out of the way.
Tell and rely on parents, relatives, friends. Is your boss decent? Tell them discreetly. If you see or speak to his friends or family don’t lie, tell them, but keep it unemotional and the bare facts. It might seem as if they support him/stay neutral, but they’ll know he’s a shit, you don’t need to tell them. Stay off social media, temporarily describe accounts if you need to.
Call women’s aid or visit the website - they have loads of numbers for free legal advice too.
If he starts shouting and creating, film him / call the police/call his parents/a friend, stay calm.
All of the above is reversible if needed, but you can’t create a head start later.
Make a list of what he’s done and look at it when you weaken. When you’re mad, make the phone calls / arrangements above. When you feel overwhelmed, call a friend. Think about the advice you’d give a friend in this situation. Have lots of fun with DC. Exercise if you can - even just a walk out with the DC, fresh air helps. Eat/sleep whenever you can.
I’m so sorry you’re going through this OP. He is an utter shit. You’re grieving for what has been lost and the false image he created of himself. This is not your fault and will pass - you will come out the other side stronger and happier. Repeat, repeat, repeat, repeat!
I’m going through something very similar and at a similar stage. Only he blindsided me by leaving before I’d realised anything was wrong and is saying he just doesn’t know what he wants and needs time - no mention of OW but I’m betting she’s lurking. I’m lining up my ducks now! And obvious, but the advice and support on here is brilliant, keep posting. Good luck xx