Doing some video review and this happened.
1:30 or so in the 4th QTR. B is up by 9 pts.
A 1/G @ B4. A throws the out, and B33 intercepts at the GL. He's taking it 100 yards and smoking the Referee in the process.
While B33 is running it back, several members of B's team enter the field. Some of them are intermingled among opposing players who now have to discern which member of B might attempt a block. One such potential A tackler may have had a chance to stop the TD, but had to alter course because of the B interloper.
The crew called illegal participation and brought the 100yd TD back.
Can there be live ball illegal participation here? In my opinion, the kids entering the field aren't A. Players, B. Substitutes, or C. Replaced Players per Rule 2 verbiage, and that eliminates illegal participation.