Whenever you buy a new pair of shoes you should know the length of the child's foot. This is important as shoe manufacturers all use different moulds to make their shoes (and these must suit their design).
Even if you have been given a measurement for you child's feet you are best to fit the shoe to the foot rather than get too hung up on what the size is on the measure, as foot measures are really only true to the brand that provided them.
The good news is measuring feet is easy and can be fun!
This can be measured easily and just as accurately at home, by you, with no specialist equipment; a piece of paper, a pen and a ruler are all that is required.
Phil (Store owner, barefoot runner, Dad)