@Siablue in cases like that, I think the child has a good start in life.
I struggled financially to begin with, but because I consciously made choices that would not disadvantage us in the long term and went without things that would only benefit us in the short term, we are ok now. I left early though, DC were young.
A good friend of mine, when she left her xh, she returned to very comfortably off parents and she and her child have a very good relationship with lots of financial and practical support.
My own parents were married and managed to do such a poor job of parenting me that when I met an abusive man 20 years ago, I felt 'ah this is familiar'. I have worked my way through that, out the other side, but married parents aren't just two people to love the child, their relationship, their dynamic dictates the atmosphere in the house. And if that relationship isn't completely healthy, respectful, affectionate, equal and free from fear, resentment and stress, then children are exposed to constant discomfort and tension that they wouldn't experience if there was one adult in their house. The dynamic between two parents is such an influence on children.
Not claiming that being a single parent is the ideal, but it is not necessarily bad either.