If your kids foot length is 15.5 cm, then add 1 cm ~ 1.5 cm, you need to choose the size which insole length is 16.5 cm ~ 17 cm.
size 6 = Chinese size 21= Insole length 13 cm, fit for foot length 12 cm
Size 6.5 = Chinese size 22 = Insole length 13.5 cm, fit for foot length 12.5 cm
Size 7 = Chinese size 23 = Insole length 14 cm, fit for foot length 13 cm
size 8 = Chinese size 24= Insole length 14.5 cm, fit for foot length 13.5 cm
Size 9 = Chinese size 25 = Insole length 15 cm, fit for foot length 14 cm
Size 9.5 = Chinese size 26 = Insole length 16 cm, fit for foot length 15 cm
size 10 = Chinese size 27 = Insole length 16.5 cm, fit for foot length 15.5 cm
Size 11 = Chinese size 28 = Insole length 17 cm, fit for foot length 16 cm
Size 11.5 = Chinese size 29 = Insole length 17.5 cm, fit for foot length 16.5 cm
Size 12 = Chinese size 30 = Insole length 18 cm, fit for foot length 17 cm
For wide foot, please choose one size bigger.