First of all, it should be noted that the inner length of shoes refers to the inner length of shoes, not the outer length of shoes. So you need to pay attention to confirm whether the inner length of the shoes you buy meets your needs.
Secondly, generally speaking, the inner length of shoes is slightly longer than the wearer's feet to adapt to the expansion and movement of feet. But for children, the feet are still developing, and shoes that are too long or too tight may have adverse effects on the feet. Therefore, it is suggested that when buying shoes, choose shoes that are slightly longer for children's feet, but not too long.
According to the information you provided, the suitable foot length of shoes is 17.9 cm, which is slightly different from that of your child wearing thick socks. If a child's feet are not particularly tender, generally speaking, suitable shoes should be wearable. However, if you are worried that your shoes are too tight or too loose, I suggest you consult a professional shoe salesman or pediatrician for more accurate advice.
In short, for the choice of children's shoes, we should pay attention to whether the inner length of the shoes is appropriate, and choose shoes with slightly longer feet, but not too long. If you have any questions or concerns, it is recommended to consult a professional for more accurate advice.