If your kids foot length is 15.5cm, then add 1 cm ~ 1.5 cm, you need to choose the size which insole length is16.5cm~17 cm.
size6 = Chinese size 21=Insole length13cm
Size6.5 = Chinese size 22 =Insole length13.5cm
Size7 = Chinese size 23 =Insole length14cm
size8 = Chinese size 24=Insole length14.5cm
Size9 = Chinese size 25 =Insole length15cm
Size9.5 = Chinese size 26 =Insole length15.5cm
size10 = Chinese size 27 =Insole length16cm
Size 11 = Chinese size 28 =Insole length16.7cm
Size 11.5 = Chinese size 29 =Insole length17.3cm
Size 12 = Chinese size 30 =Insole length18cm
Forwide foot,please choose one size bigger.