Start with some stretching and finger wake ups...
Gently stretch wrists, flexing back and forth... then wrist circles, 5x in each direction...
Finger massage... massage each finger with he opposite hand. Younger kids are not always "in touch" with their digits! (har har)
Counting: Touch each finger to the thumb... Start with the pointer and thumb and then count "one", on to middle and say "two", etc. on down to the #4 pinky... this also teaches the numbering of fingers used on some chord charts.
Count forward and backwards while you tap: "1-2-3-4... 4-3-2-1"... then get random, and repeat the same finger sometimes... "1-2-2-4-3-3" and such. Go slow at first, then faster. Try to have them tap their thumb and fingers together hard enough to hear with their hands by their ears... builds hand strength!
Once you get good at this add this challenge: Go 1-2-3-4 on one hand and 4-3-2-1 on the other hand... we call this "Opposite motion" and it's quite difficult for some (like me).
Put all five fingers together with the five from the other hand, and then tap each together four times: "Thumb thumb thumb thumb... pointer pointer pointer pointer..." and so on.
Touch hands together behind back, and switch directions going over the shoulder on each side.
Imaginary weight lifting: use muscle resistance to "lift" imaginary kettle balls.
Shoulder rolls, front to back, reverse directions, then seesaw from right to left.
You can add even more if you have time... we ALWAYS stretch and do these body awareness exercises before I start teaching and playing with my sixth grade students...
Have fun! It's really rewarding to teach kids how to play!