It's not supposed to be waterproof

When I was a swimmer, We used to put baby powder in our caps before putting them on to make it easier to adjust. It may help with the rising up.

My daughter wears the regular rubber ones for swim team. The silicone ones don’t seem to stretch as much. They are designed to keep your hair in one place, not to keep your hair dry. Although, when using the cap her hair should not be soaking wet when he gets out of the pool. Most girls will wear a ponytail or bun at the top of their head. My daughter doesn’t have as much hair so she wears a low ponytail. She normally bends her head down and holds front of the cap to the top of her forehead while I quickly stretch it over her entire head. We never seem to have an issue of the cap riding up.

you need a long hair swim cap-
This one works really well for my AA daughter with lots of braided hair.
https://www.amazon.com/gp/product/B07PRYML4X/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1
for unclear reasons to me it is showing as a paperback book when I try and link it- but this is the name of the cap-
GoodByeReality! Long Hair Swim Cap Women Swim Cap Extra Large Swimming Cap Shower Cap [Dreadlocks, Weaves, Hair Extensions, Braids, Curls & Afros] Women & Men Silicone Caps

my dd is a similar age to your dd.

A bun doesn't work as well. braid the hair in one or a couple of braid so it's as flat as possible against her head. Less riding up.

OP. Thanks. I understand that none are truly waterproof, but I’ll settle for somewhat waterproof, meaning not those stretch knit ones.

None are fully waterproof.

Silicone. Try a bunch and see what brand she likes best.

I'm not aware of truly waterproof caps. Caps are more for containing hair than keeping it dry. The plastic rubber ones have to be so tight to keep even most of your hair dry.

We like poolbeanie, you can get them on Amazon. They fit well and don't pinch. But they aren't waterproof.

Dear parents, this may be the most stupid question ever, but can someone recommend a waterproof swim cap for a 6 y.o. girl with long hair? It seems that every single one I bought rides up when I put it over her hair bun. They all say “one size”, so I guess it’s not a sizing issue. What am I doing wrong?

TIA