The classmate's testimony is exculpatory for Terri, and leaves the door open to anybody and everybody being a perp. There's no way LE is going to acknowledge it unless/until they've charged someone, and that someone is other than Terri.
Should Terri be charged, LE won't acknowledge it, but we'll probably see this child on the defense witness list.
The only thing that can dispute the child's testimony is another witness who saw Kyron after he did, and they don't have that.
I'd be willing to bet that the prosecutor told them not to acknowledge the child in case he/she has to take Terri to trial. Otherwise, the prosecutor stands too great a risk of losing his/her case, and may not even be able/willing to charge Terri.
This is very much worthy of a thread of its own. That one tiny little short interview by that one little child has presented LE and the DA significant problems.