I'm sure you are aware how shoes can vary in size and conversion size (US, UK, EURO, cm) between one designer and another in Adult Shoes. So far, I have found that is not so much the case with children's shoes. When I list my kid's shoes for sale, both Youth and Toddler, I will list the USA size, the insole length, and a Euro, UK, or cm sizing if available. When I list the insole length, I measure from the toe tip to the heel edge. Because children come in all shapes and sizes, I do not compare any ages to any sizes.
While I do not have any children to actually try the shoes on and compare, the measurements seem to mostly be them same with a variance of 1/8 inch or less. Listed below, is a table of the sizing and measurements found.
Please note also. Toddler sizes are listed from size 5 to 12, and Youth sizes from 1 ½ to size 6. I have found some Designers will put size 12 ½ as sometimes Toddler and sometimes Youth. So my store categories will have size 12 ½ in both Toddler and Youth sections.
I've also noticed that some athletic shoes, such as from Timberland or Merrell, could easily be for Girls and Boys, so I will usually place them into the store under who I think would be more likely to wear them. If you are looking for athletic, casual wear, keep that in mind.
Below is a list of general sizes, measurements and conversions. Some sizes are less than a ¼ inch in difference so you may see some the same measurement from a size to a half size. Not all designers follow this chart!!! It is based mostly on my Kenneth Cole Reaction, Timberland, Sperry, and Sam & Libby. Merrell is slightly different in EU conversions, and Dr Martens is a whole other subject.