I would use an online chart if you have a printer and find anything. I don't see anything on Keen's site, but, I'd call/email to see if there is anything I'm overlooking. My son was wearing an 8/9 at around 18 mos . . . which is large for his age. He's 21 mos now and his size 8 boots look too small (like his toe is right up on the ends) and his size 9 shoes (different makers) look like they're fitting well with a little room in the toe (as he should have).
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My first tip off that I was raising Big Foot should have been when the first walkers I bought him didn't fit when he was 12 mos old. He wasn't with me and the sales person advised that the shoe size was "normal" for his age.
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FWIW, my daughter's Keens seemed to run true to size.