Sizes marked C are our smallest boots, and equivalent to toddler (T) sizing in the US.
The sizes that are marked K are youth/kids boots that are smaller than the adult sizes (except where they overlap - a 7K is the same as a 7 Mens) but are larger than the toddler sizing. The K sizes are the same for youth boys and girls across the board, which means that they will run larger than a girl’s size in other brand shoes.
See the size charts in each model for the Mondo Size, which measures a foot's length in centimeters and spans all sizes.
Also, our Mini-Grom & Grom Boa boots offer size markings on the bottom of the outsole. So if you can find a pair in a local store, you can hold the bottom of the boot up to the child's foot and see how it will line up with the internal fit.