I think a six-year-old could probably deal with "Dee and I are having a hard time being friends right now, so I don't want to spend time with her. I'm afraid that means no playdates with her daughter, and that really sucks and I'm sorry". If there's joint custody—which presumably there is if this is a simple case of infidelity and the husband wasn't also harming or neglecting the child—then the LW can also say "You can see Dee's daughter when you're at your dad's house".

Making new friends sounds ideal. I feel really bad for this kid, especially if she grows up and realizes that the reason she and her best friend get to be stepsisters is that her stepmother is the reason her parents broke up.