Every child is different, every foot is different.
We suggest purchasing the closest size applicable, and allow you child to grow with M1B. If money is tight, then opt for a size bigger than their size now (if they are a size 7-UK now, then purchase 8.5). This will give your child more time to grow into the boot, giving them more use from the boots. Have your child wear 2 socks on each foot at first till their foot grows into the boot. Generations have used this cost cutting choice.