You can find handmade Apple Watch bracelets on Etsy. Alongside a lot of dropshipped crap ones from Aliexpress - but the point is it seems like third parties are able to make bracelets that replicate the mechanical “interface” for the bracelets, so you should be able to find any size you want.

Many of my daughter's 11-year-old friends have Apple Watches as their mobile device. Some of them have tiny wrists.