All sorts of situations (and apologies I've not read 196 posts before adding my twopenneth) but when I saw - "What I want to shield my daughter from is the knowledge that some dads and mums don't want to be a part of their children's lives. That must be hard for a child to take?" - I just had to add:
There are lots of reasons why one parent is not part of their child's/children's life... In my case, my father died when I was 6. Fortunately, with three elder sisters and a half-brother I had plenty of family and hardly noticed... (Mum was first widowed at age 19 in 1939, then widowed second time in 1965.)
So yes, for me, it does seem a bit 'precious' to expect her to believe all children have Mum + Dad to love and care about them... while separation and so on are an unhappy part of life, it is sometimes for the better and both parties move on... one partner dying is clearly not desirable, but compared with hostilities between separated partners, might be a 'simpler' situation.
As for what step-mothers do in fairy tales, I've no clue, I'm sorry.
Society is far more complicated these days than M + D + 2.4 children and it's not something to get 'upset' about, but some people have 'different' living arrangements, and so long as there's love and happiness, they should all be acceptable.
As for discussing situations where there are rows, or worse, violence, that's a whole different ball park... and surely 'separated' is a better situation to stop suffering, so worth including in family scenarios for your daughter, IMO.