Unless they’re really, really careful, they can’t call either parent. As soon as they say “good boy”...they’ve opened the door.
And it would take all of about 30 seconds for the defense to get them to say that.
If the direct testimony is about whether those are Martin’s screams, then I can picture the following question on cross:
“You have stated that the voice heard screaming was TM’s. We have previously heard testimony that even a person well acquainted with the screamer must be familiar with the screamer’s voice under similar circumstances - in fear for their life or safety. Can you describe to us the situations under which you have heard TM scream in this manner so we can understand your basis for making this judgment?”
Either the parent dimes on the past history of fights, or claims selective omniscience, neither of which is helpful to the prosecution.