While I don't directly have that issue about casting family, I have dealt with this issue before. My producer's and helper parent's child(ren) have been cast before.
The thing I realized is, unfortunately, you can't stop people from saying what they are going to say. However, I have always made known the following to everyone at the parent meeting in writing:1. Everyone is given a fair shot at casting2. While rubrics are used at auditions and crew interviews, there are other factors that go into casting a show.3. I encourage you to support your child no matter what role or position they receive4. I empower your child to discuss with the director what they can do to improve their work.
While I know this won't stop everyone from saying anything, unfortunately sometimes it's just the way it is.
All the best!