The assistant can and should make the call on the out of bounds call. The head official can choose to accept it or not. The reason for the assistant is to get the calls right. But the assistant cannot blow the whistle and if the action is fast moving when the infraction occurs it isn't unusual for some time to run off. Also, the assistant knows that the head official might disagree so he isn't going to be jumping in and stopping a takedown in progress in case the head official sees it a different way. If the kids were on their feet, the assistant no doubt saw them both step on the line...they are out...the assistant's job is often to "watch the line" while the head checks out everything else. I don't think you can read anything into "body language" or anything else, obviously the head had confidence in his assistance or he could have ignored the advice.

An off mat official isn't going to jump into another official's match to report an infraction, such as punching, unless that mat official asks. Remember too, a lot of these officials are working with guys they don't normally work with so you don't have the same familiarity as you might otherwise, say for a league tournament.