Mute strumming is very effective for teaching various rhythmic strums. I don’t think we spend enough time working on right hand strumming because we are so concerned about left hand notes. Isolating the right hand helps the students advance in their ability to keep a beat and learn complicated and syncopated strum patterns.
Teach the children how to play a mute strum by lightly placing all left hand fingers flat across the strings around fret 5. Have them watch their hands as they do four down strums. Ask what is happening in between down strums and let them discover an up strum that is silent. Tell them that their strum arm is like a windshield wiper that never stops. Call the down strums quarter notes and down-up strums pairs of eighth notes. For longer notes pulse the hand mid air to show the beats that are not strummed.
Next put easy rhythm patterns on the board and have the students guess which measure you are strumming. Have them imitate you making sure that they are keeping their strum arm moving steadily without pausing.
The next step might be for students to create rhythmic patterns and notate them so they can help their peers learn them.