Kurt , so sorry he turned out to be such a knob. I fully expected him to just come home and act as all was well, glad for you that that is not the case. Like others have said, lucky you didn't get as far as marrying such a twit. In Australia he would be akin to a 'toolie' (basically a tool, an idiot 20 or 30 something who tries to hang out with the school leavers (schoolies) every summer, they often get arrested and kicked out of the young'uns areas by the cops, the losers!).
You sound so grounded and lovely I have no doubt that you will meet someone lovely to share your life with, you aren't available for him if you are tied up with your fuck knuckle toolie.
Good advice given. I know you didn't/don't want to split but I think 'hope for the best, prepare for the worst'. in addition to legal paperwork get all of your baby photo's and memory cards and leave them at your Mum's.
Ime when men are caught in the wrong they can turn really nasty, protect yourself and your DCs.
If he was to return to you in a positive way he would need to do a massive rhythmic gymnastic type sustained turnaround in his behaviour.
If their is an OW possibly he will have his fun while you suffer and right about when you are starting to gain strength again, she'll dump him and he'll come crawling back. Maybe you should think about how you would deal with that.
for your Mum and Dad, poor things, it will be hurting them to see this happen. You have good support with them.
CSA will take a massive chunk of his pay like someone else said, bit hard to party on that. I bet he has a good run of partying then crawls back with the usual shit about you're the one, he wants a family, OW tricked him, has a problem with drugs/ alcohol/ gambling blah, blah, blah etc.
The best revenge is success. Thinking of you. You're babies have a rather childish father but they also have beautiful, mature, SuperMum also!
UMN Hug.