Okay, you have a better perspective than I do. I have not watched them on stage very much. Just brief snapshots, here and there. I was unaware of their behavior.
But still, remember they are kids whose lives have been tuned upside down. They are traveling in and out of multiple time zones, being forced to dress up, surrounded by strangers, and cameras, all negatives.
Few kids have a good stage presence. They should have kept them offstage and pulled them out only for the intros and the finale. That would have been easier on them.