This is a nice little game that we learned in the Nordoff-Robbins improvisation session;
The leader establishes a steady pulse on their instrument, and everyone joins in. The rules are dead simple.
When the leader calls “Hey!” everyone switches to “twice-as-fast”.
When the leader calls “Ho!” (rhymes with slow) everyone switches to “half-as-fast”.
So, assuming that you were playing “crotchets” to start, two “Hey!”s in a row will have you playing semiquavers.
And two “Ho!”s would make everyone play minims, and then semibreves. Which was surprisingly difficult!
I suspect the children would have to be fairly experienced at playing simpler pulse and rhythm games before I tried this one. But we all seemed to enjoy it!