Freeze any joint bank accounts so he can't empty them.
Contact CAB and council - get advice from them regarding housing, council tax, benefits, etc....
Get documents safe. Passports, birth certificates, marriage certificate. Get any proof of income you can. Bank statements.
Get to a solicitor for some advice, a lot give a free half hour.
Look after yourself. You will be no good for your kids if you are running on empty.
You may not be able to eat solids (I couldn't). Sugary tea to keep you hydrated and your sugar levels up. You don't want shock on top of everything else. Ice lollies. Sounds mad but they work and hopefully won't make you feel sick. Soup and smoothies.
When you can some toast and butter.
Cry when you need to. You may run on adrenalin for a while but you WILL crash at some point. Be ready for it.
Contact family and friends. Get them to rally round and support you and help you as much as you need them. Believe me, they WANT to help. Do NOT go through this alone. It's a nightmare. You will feel relief when you tell people. Tell people why. HE was unfaithful and is a cheating scumbag. The more people that know this up front, the better. Don't keep his dirty little secrets. You owe him nothing. NOTHING at all!!!!
Find out how much maintenance he should be paying. Do NOT agree to sign anything without legal advice.
I'm so sorry this is happening to you. The list of things to do will be endless but please please just take things slowly. For now only think about things an hour at a time. Look after yourself and look after your kids.
Get RL support ASAP!
for you.