Here's what you need to know: ski boot sizes are expressed in Mondopoint
Note that the size of ski boots is different from that of conventional boots. It is expressed in "mondopoint", which simply corresponds to the length of the kid's foot in cm. For example, for a small foot 23 cm long the ski boot size to choose is 23.
Do you know your kid's shoe size? Refer to the size chart to find out their mondopoint size.