Obviously, we are interested in the inner dimensions of the shoes, as well as the width and length of our children's feet with an accuracy of millimeters, but here too it is necessary to proceed with reason. Recently, the inconvenience in the selection of children's shoes has been linked to the millimeter differences, which are often an obstacle to buying or even an incentive to guess and direct.
First, it is necessary to say that we evaluate the length and width of the foot (at the widest point). According to the measured values we choose the right shoes because we know that the ratio of length and width can vary considerably between manufacturers. It used to be that we wore too narrow a shoe on the wide foot due to lack of possibilities, thereby narrowing its support base, limiting it in function, and often irreversibly deforming our toes. In a too narrow and pointy child's shoe, the thumb usually suffers.
Nowadays we know what parameters we expect from a child's shoe - it is above all a suitable bending stiffness with respect to the age and weight of the child, the same height of the sole under the toes as under the heel, the absence of longitudinal arch support and shape respecting the contour of the foot. To the measured value we add only the length excess, which again, as we already know, choose according to age (fitness in walking) and possibly the season or style of shoes.
If the shoe has a removable inner insole, it is convenient for us to verify the shape of the foot and enough space just for the dominant thumb, but judging by the dimensions of the insole size of the shoe is misleading as well as the myth of oversize "wide".
The upper of the shoe is always partially compliant and the shoe then provides more comfort than might seem from the dimensions of the insole. The shoe should not be too narrow for the reasons described above, nor too wide, but the difference of plus or minus a few millimeters is not an obstacle.
Recall, a few years back we could choose from a narrow range of children's footwear without knowing the internal dimensions or shape differences, often with the possibility of trying a limited number of models from mostly domestic manufacturers. Today, we have a wide range of shoe width and length ratios in a variety of shapes to reliably wear our children and ensure proper foot function and development.
At this time, aren't the conjectures about the millimeter differences reported by each vendor a bit inappropriate?