Can I say "elle me parlera d'eux demain" here?
No, you'd probably use eux only when talking about people in this situation.
Sandrine a des enfants et elle me parlera d'eux demain.
I can't see see what I have done wrong as it is exactly the same as the answer
i wrote word by word the answer and did not accept it Lea a des problems et elle m'en parlera remain What is wrong with my answer?
Problèmes and demain. You wrote problems and remain.