When single Moms have an S.O. that becomes a father figure to their kids, then the relationship fails...?

...can the kids still be better for having known the man? This question is close to home for me, because 2 years after breaking up with my ex, I now miss her kids more than ever. She specified that I won't see them after the breakup, and maybe that's for the best with how cold I would have been to her in front of the kids. The thing is, I was really good with & to her kids, better than her in some ways. They lived in my house for 3 years. They saw the demise of the relationship (fighting, me leaving for a few days, etc...), but there was no physical violence. I guess the bottom line is whether the bad parts (seeing the fighting, abandonment) outweighs the good influence?