Usually they don't allow witnesses to sit in the courtroom----EXCEPT when the witnesses are parents of the victims. I have seen the judge make exceptions in their case because they obviously want to see the proceedings.
I think it may depend upon whether the judge believes one of the parents would be influenced by the other testimony before they gave their testimony.
In this specific case it could be problematic for Billie to testify as well as see all the trial because of her very close association with the defendant.
Some might even think she should be sitting next to him at the defendant's table...