I have, or have had, cPTSD. I don't mind who knows, as it isn't a source of shame to me. Strictly speaking it's not a mental health disorder: it's a psychological injury. It's also a textbook, human and very normal response to extreme trauma. Although it can make you very physically unwell, it's not so debilitating that it precludes you caring for yourself or dependent children. It's also eminently treatable (EMDR therapy has changed my life).
It's very doubtful a court would remove children from your care for this reason, and it's also likely that if a former partner tried to use this against you, they'd recognise this ploy for what it is. Don't forget, they see this every day. That said - as a former sufferer myself I'm well aware of the kind of abuser who would use this as a tactic against you. If you're thinking of using the PTSD diagnosis as a means of making him back off, don't. Here's a story of what happened when I did this.
I grew up with an abusive father (one of the main reasons I've carried the condition around with me for so many years). He was never in my confidence but he was the first person who learned I had been raped at 15. I finally broke, after having him constantly in my face, needling me, pinning me up against walls where I couldn't run away so I'd be trapped while he sneered unpleasantries in my face but he wouldn't leave a mark. I was in that position one day two years after the rape and threw it out at him in desperation as the only means I could find to make him back off and leave me alone. It felt like my only escape mechanism.
He went absolutely ballistic. First he victim-blamed me telling me it was my fault because I was a slut (as if he'd know). Then he told me he didn't believe me anyway. Then he assaulted me. Then he tried to call the police, which I didn't want because I was afraid the whole street would find out and laugh at me (my two rapists had mocked my anatomy and made me feel like a freak). Also, my memory of the event was hazy (I later learned this too was likely a response to trauma, but with two rapists' word against mine and the whole event partly blocked out by hysterical amnesia, what chance would I ever have had in court)? Then came his trump card: if the rapists ever did it to anyone else it would be my fault.
The reason I'm posting this info here is borne out of a strong conviction that it's never a good idea to give an abuser leverage. Once they see a vulnerable area, they don't let up. Don't tell him. It's not going to make him back off, and if you're as unfortunate as I was it will have the opposite effect. But nor do I think you should fear that if the diagnosis did become known, your children would be taken away.
I hope you can get the help you need. It's a truly dreadful condition. It seems impossible to see the end of the tunnel when you're in that situation, but with time and the right treatment, you will. If I did, anyone can.
