It depends on the age of the kids what to tell them. Agree a routine so they know exactly when they will see him and him sticking to it is the most important thing, so there is some certainty in the storm.
Prepare yourself for getting all the anger and upset from the dc flung in your direction. if he's the one leaving they wont rock the boat with him as will be terrified of losing their Dad. You are their rock and safe space and will get all the emotional outbursts. Its not fair and you just have to keep reminding yourself its because they trust their relationship with you.
Find an outlet - friend / family who can support you away from the dc.
Go on gov uk website about separation and moneysavingexpert / benefits calculator if relevant and work through your finances systematically so you are on solid ground for the short-term. You can then gather evidence and get advice about the longer term but need a plan for immediate bills and protecting yourself from debts he may run up.
I use the CMS for child maintenance as I just prefer a third party deal with ex and collect his financial info
Its easier for you to minimise contact. Maybe use a parenting app. Its difficult if there is constant back and forth contact you do need space to just absorb and think. Block him on social media etc remove any ability to track what he is up to - you will be tempted to but its better not to know.
Book a nice holiday / something to look forward to.
I found redecorating really therapeutic - ex was controlling and i erased his influence from the house.
You may need STI check. Think about counselling / therapy for you & / or kids.
Tell school so they can keep an eye on your dc and cut them some slack if they behaviour is out of character.
Be careful not to tie yourself into childcare arrangements which prevent you earning or put all the costs on you. Taking on weekdays puts all the costs on you and also limits your earning money so its a double blow - while he earns fulltime and does some weekends.
Do not underestimate the ability of a selfish man to walk away from his kids and responsibilities. This is often the most shocking thing, but it happens all the time, even with previously hands-on Dads.