Entirely black, not white or other color, sneakers are available from New Balance, Nike, Reebok, and most other sneaker brands.

If the rule is simply all-black, then go with one of those. If the rule is all-black AND no-sneakers, then find something simple and affordable with good cushioning.

My son wore all black tennis shoes, they are actually often called uniform shoes in a variety of brands. This was K to 3rd and they were all velcro or slip on and comfortable. Check zappos. I think new balance and saucony were good ones.

Older kids mostly wore Sperry or loafers.

/\ I assume you meant for boys. For girls there are tons of Mary Jane style with sneaker soles.

Doc Martens are also good, but pricey.

Would something like this past muster?

https://kizik.com/products/kids-prague-blackout?variant=42780340355229

How specific are the guidelines? My kids were able to wear all black sneakers. Very all black. Nike makes them. They flew under the radar. It really is difficult to find child "shoes". Finally the school relented and sneakers are permitted in general. It was actually becoming a hardship in the population to find "shoes" and keep replacing them as kids' feet grow quickly, and the shoes aren't really good for any other uses. And by hardship not only $$ but silly time-consuming.

My kids will be attending an international school next year that requires uniforms. For shoes, they just need to wear black shoes (they’ll change into sneakers for PE). Any suggestions on comfortable uniform-appropriate shoes? They are not excited about this part- they live in sneakers and crocs.