Look for a bra with a J-hook at the back.
Strap positions shift depending on how low/high you slide the hook.

Look for a balconies or plunge style.

I have a couple of deep square neck tops and this T-shirt bra from Third Love fits perfectly underneath. It’s also my go to daily bra.

https://www.thirdlove.com/products/24-7-classic-t-shirt-bra-taupe

How low is the neckline? Does your DD have a bigger chest? I can’t imagine a square neckline being an issue.

Try a “balconette” style. And Her Room bra shop used to have a virtual try on with different necklines that can be helpful

Has anyone found a comfortable, non-strapless bra that works with square neck tops? Either for yourself or daughter. My 13 year old asked me for help with this tonight and she never asks for anything so I’m on a hunt. Thanks!