Polly died in the train crash with the others and wound up in the True Narnia. BTW she never married, in the Last Battle they call her Miss Plummer, and Plummer is her maiden name.
Polly and Digory did keep in touch over the years. She does appear in The Last Battle, but you can see that the kids all got to know her before then because Eustace and Jill refer to her as Aunt Polly even though she isn't their aunt. They're obviously close to her in LB. It's implied that all of the younger ones called her Aunt Polly so I'm guessing that she was in touch with them even though it never says that they stayed with her like they did the professor.
She did still go by her maiden name implying that she never married. She did reach the New Narnia with the others as well.