Guide the kids into proper posture without unneeded tension. (important at least to suggest as it will hopefully help them develop the habit of healthy posture during all activities, not just meditation over time)
Give them the arms-length-thumb-high-as-top-of-head target: Have them extend one arm, make a fist and hold the thumb pointing upward and raise it so the thumb is even or slightly higher than the top of their own head and look at it to get used to this position and then close the eyes and pretend they are still focused on the thumb. This is the eye position for closed eyes during most all meditation exercises I would recommend. Or explain it some other way, like “imagine you are looking at a star just over the horizon at night.” This isn’t exactly the same but I think it’s close enough.
If their faces look contorted ask them to relax their faces and eyes.
Guide them through a few slow deep breaths (this can be omitted or greatly expanded depending on the age and energy of the group, time available, and what the leader is trying to accomplish)
Ask them to begin listening in their right ear for whatever sounds they can hear.
Then explain you will make a noise and they are to listen to it very carefully (especially in the right ear) then softly begin to play the bowl but try not to bang it, or play it too strongly. Remember that kids can hear things that adults can’t. So after you stop hearing the tone, the kids may continue listening for a time, so don’t interrupt them. Let them listen as long as they can hear it.
You’ll have to decide how long and in what way you want to play the singing bowl. Eventually though, let it fade out into nothing and meditate on the fading sound with the kids and ask them to enjoy the stillness for a moment or two if you like following this exercise.
There are many types of singing bowls and other sound makers that can be used for this meditation. I like using a large crystal bowl because the sound lingers for a good amount of time.