If it’s possible, trace your baby’s feet on a piece of paper. May be easier to measure it with a ruler on a piece of paper than on a squeamish child 😅 there are also some printable shoe charts online where you can just place your baby’s feet on it and it’ll tell you the size. Here’s a link! https://www.care.com/c/stories/4514/baby-shoe-sizes-what-you-need-to-know/en-au/
I bought this on shopee hope it helps! https://shopee.sg/product/77581379/6142331765?smtt=0.4182125-1628733934.9