Dear Winger,
If you know them, the PNF trunk patterns are great fun. Have used them a lot and they are very effective in improving strength.
Another way is using roller boards, you can let the child roll around on his/her stomach, back, kneeling, on all fours, lots of fun stuff.
Using certain yoga exercises and exercises used in karate and dance are fun for kids as well. And there's lots more....... use your imagination and make fun games ...
good luck
Esther