Picking out a kids barefoot shoe size can feel daunting. But because they are growing (and each brand's sizing is different) it's always best to use current foot measurements to select a size based off the brand's size chart. You want your child to have a minimum of 12mm extra length in their shoes, so that means new pairs can have up to 17mm extra length for growing room.
Here are my best practices for measuring kids' feet:
Measure twice just in case. Measure both feet and order based off the larger foot. Measure in the evening when feet are largest. For wiggly kids or very young kids, it can be helpful to tape your paper to the floor. Make sure your child is standing and putting their weight on the foot you are measuring. If you plan on having them wearing socks, make sure you measure with the socks on. Add about 15-17mm to foot length to find the internal shoe length you should aim for (plan on less than sandals, and more for boots)
Below is a video that explains two ways to measure your child's feet. I like the wall method best because it is most accurate, but I usually do both to compare.
Once you have your measurements, read this FAQ on how to use a size chart to select a size!