Let him go, let him see that you're not falling apart at the prospect of his departure, and put the most cheerful spin you can on his leaving in order to mollify and maintain stability for your dc.
While you're presenting an unaffected exterior to the members of your household, rant, rail,, and call him all the names he deserves under the sun here where others who've been where you are now will get you through it and enable you to have hope for the future.
Start preparing for the inevitable by consulting a rottweiler solicitor who specialises in divorce and family law - many offer a free half-hour consultation and you should try as many as is required to find one that you have confidence in.
If you feel yourself beginning to crumble make an appointment with your GP, ask for a referral for whatever talking therapy is adjudged best for you and, if you feel it's needed, also ask for a short course of antidepressants and/or sleeping pills to keep your equilibrium on an even keel.
If your h's disaffection is a fairly recent development don't be surprised if he takes up with an ow shortly after leaving as it's probable she's lurking in the wings waitingfor her turn in the spotlight of his radiant countenance
Courage, ma brave. Even though you may not believe it at the moment, you've had the best of him while your best is yet to come.