What I would say is, “this is an issue we as a family care deeply about. Your mother and I will be going to talk to our representatives to discuss it on behalf of the family. We’ll let you know how it went when we get home, hang out with Granny and we will see you later tonight.” Leave your kids out of it, no good can come to them speaking out on an issue like this.
“Leaving your kids out of it” simply means that as a parent you really don’t believe in what you are telling your children about what is right and what is evil. This young girl has more sense and courage than you do. She definitely knows the difference between right and wrong. God bless her.
That would work fine for something like a legislative hearing, where the parents can represent the family - which is what you were objecting to.
Would you tell them to refrain from commenting about the issue in the contexts it comes up in for them - stuff like classrooms, Facebook, etc, where their identity is known?
Is it the wideness of the audience and public access to the comments, the lack of anonymity, or something else that bothers you about this girl testifying?