What a sad situation to be in.
At the moment your DiL is very angry and looking for people to take her side against your son, no doubt so she can throw it back in his face!
So your son didn't ask your opinion - why should he? It is his marriage , and when all is said and done, it is up to him how he deals with it. However, it wouldn't be a bad idea for him to sit down and have a talk with you to explain what he feels has gone wrong , but as a bloke, I doubt that will happen at the moment- in my experience they make a decision, and that is the end of it !
I think you need to meet up with your DiL away from your home and tell her calmly that although you care about her (even if you're lying!) , what has happened is between her and your son and even if you think he is an idiot, you will not take sides. Now, how can you help her with the children ? It was a tactic my mum used, several times over - there for the kids, adults could take care of themselves and it seemed to work.