When looking at purchasing or hiring ski boots for kids, you'll need to understand ski boot sizes. They are measured in 'Mondo Point' sizes.
Below is a chart that reflects Mondo Points in comparision to a normal shoe size (in US measurements)
Don't worry if you're not familiar with US shoe sizes as we also have another chart below which is a comparison in shoes sizes for around the world.
Please note this is only a guide and whilst 'Mondo Point' sizing is universal you may still find some brands may choose to have their own size chart.
All that said this is a great place to start.
Good luck, and remember, a well fitting ski boot is an essential requirement for a great day on the slopes.