This is a bit of a golden oldie, but none the worse for that.
Working in pairs, the children mirror each other’s movements; child 1 is the leader, and child 2 is the “mirror”.
So, whatever child 1 does, child 2 mirrors it exactly. It is easier to demonstrate than to explain, so I suggest that you show the children what you want them to do, rather than waste too much time explaining.
Swap over the roles from time to time, and also pause a few times to let the children talk about the activity; what kind of movements work best? Why? What skills are they using? If you spot a pair doing something different, or innovative, or working exceptionally well together, ask them to show their work to the class.
Once they have all tried leading and being the mirror several times, bring out the instruments. Give both children in the pair the same instrument, and try the same activity but now playng their instruments as well. At this stage, some reminders about being careful, watching what they are doing, keeping their distance from other pairs. is probably a really good idea.
It may be easier to start with everyone sitting facing each other in pairs to start with before adding movement. Again, pause a few times so that you can discuss the activity and showcase anything of interest. At the end you may be able to video and watch the children’s work, and let them comment on what is effective, and what needs re-thinking.
I tried this activity in a year 3 djembe Wider Opportunities class. We set the djembes down on their sides on the floor and sat on them, like riding a leg-less horse, facing each other. I watched while the children worked with good focus and concentration, incorporating different sounds, and rhythms and varying the dynamics and tempo with increasing complexity.