I often remind the children to "turn on your listening ears". If I see their attention is elsewhere I'll say, "Oh ladies and gentlemen, it seems that our ears have gotten turned off. Can you please turn on your listening ears?". Then we will all give our ears a tweak to turn them on. It's cute to watch the little ones "turn on" their listening ears.
It works in JR Primary. Sr Primary thinks it's silly - it doesn't work as well with the older kids - but sometimes I forget who I'm talking to and say it anyway and am met with eye rolls. Oh well.