You helped to dissolve a family.
I know that he is more responsible and if it wasn't you, it would have been someone else, blah, blah, blah. But it WAS you. You have absolutely no idea of the depth of pain that you probably caused his wife and children by screwing their husband/dad and essentially being the catalyst of him walking away from their daily lives. But whilst it's alright for THEM to suffer at your hands, there's no way YOU should be suffering, right?! Your feelings are more important, right?! She needs to fuck right off out of it doesn't she! Never mind what his daughters might need want. It's likely you'll be telling them to go to hell next - probably encroaching too much on your time with their dad!
I'm sure he is more in love/lust with you than his ex wife. I'm sure he does only view her as a friend. The spark doesn't last forever (a warning you should heed) and if his head is that easily turned, it's probably a good thing they split up. But if they're amicable then that's the best thing for their kids and you should butt the fuck out. Actually, if you force him to cut her out of the WhatsApp group, not only will his daughters resent you (and him) for daring to think you have a say after meddling in their family and causing it to implode. But he will also possibly start thinking about his ex wife MORE, if he has no access to her thoughts and feelings and sound bites. She will become mysterious. (Mystery is always attractive, isn't it)?! And at that point he might start to have some regrets...
If you want to "win" that badly then you should leave well alone and let him decide what's right for his ex and kids - after all, he upended their lives for you and if he feels guilty and treats them better because of it, then that's the least he can do!
There's no point getting all put-out and possessive now; play stupid games and you win stupid prizes. And you won yourself a good old booby prize here; you're with a man who happily walked away from his family to be with you. I'm sure that made you feel very special and powerful and fateful at the time, but it's now sinking in that you could be on the receiving end of this one day. And it's scary isn't it?
Feel that fear. You deserve it.