As a child, both my parents were 'crap' in different ways. But dad was far worse and in more obvious ways, especially when his drinking got much worse and the aggression got worse and violence increased. This happened when I was around 12/13 but my siblings were younger and also noticed it at the same time.
I've been lc and nc with both my parents at various points. Currently lc - very complicated. My siblings do generally resent that mum stayed with him.
As a parent in a similar situation to yourself - the big bolt for dd was when doing sex ed at school with backup from home and working out her fathers 2nd child was born 7 months after we split! She'd have been 9/10? She had smaller bolts before that wrt his treating her as lesser than his 'new' children, slagging me off to her, quizzing her about me.
I had not spoken negatively about him to her thinking that was the right thing, she has since and now says that was the wrong decision as far as she's concerned. She feels I lied to her and it made it harder for her to speak honestly, but negatively about her relationship with him.
As a result of ex's lack of effort dd now hasn't seen her dad for several years and is very hurt that he's basically all but forgotten her. I could cheerfully kill him for that! I try to content myself that when he's an old man (and wife 2 has probably left him and those DC having little to do with him too - I hear things, apparently not a great dad to them too) he'll regret it and by then it'll be too late.
I have no idea how to help dd though, I try to support, commiserate, distract but at the end of the day I could be the best mum in the world (I'm so not!) it still wouldn't make up for the lack of a dad.
He made very little effort even in the immediate aftermath of our split, I basically dared him into court to get contact arrangements formalised and in an attempt to motivate him to see dd, bent over backwards to facilitate contact even when we ended up hundreds of miles from each other (he was army) basically spending the cm on travel so dd could see him. With hindsight I think I should have just let him fade out of dds life. I think if I hadn't pushed he'd have done so before she was 3 and she'd likely not even remember him - and you don't miss what you don't remember ever having.