dear physio99,
In my opinion giving normal ( adult ) exercises to a 6 yr old is not functional.
To answer you question.... how are his feet... do they need inlay´s? is he hypermobile? is he generalised hypermobile? does de knock knee still exist when he jumps or only when he carries weight?
Check the growth charts for joints, the smaller child has O-knees 1st, later X and later they straighten again.
Try using games to exercise him is the least you can do if you feel he really needs therapy.
kind regards
esther