How to track a lost iPhone without access to iCloud account

If her Apple ID is locked, most likely it's because someone who didn't know the password tried to get in. See:

If your Apple ID is locked, not active, or disabled - Apple Support

There will be no way to try and track the iPhone or put it into "lost mode" unless and until you can get access to that Apple ID. If Activation Lock was set on the iPhone prior to it going missing, then no one else can use it. If it wasn't, then someone could potentially erase it and set it up as new, but all data will be gone.

See:

If your iPhone or iPad is lost or stolen - Apple Support

Regards.

jenipooh722 wrote:

I tried to log into her icloud account to find her phone, however, the account is locked.

This is a very bad sign regarding the likelihood of tracking. I suspect there’s a small (minuscule) chance that if your niece were sharing iPhone location with someone previously, that person might be able to locate the iPhone. Otherwise, without unlocking your niece’s Apple ID, there’s no hope.

If your Apple ID is locked, not active, or disabled - Apple Support

Hopefully, your niece was using a passcode to log on to the phone, so assuming that this is true, there is no way that anyone could ever guess the correct passcode in less than 10 tries without being completely locked out.

The data on the phone is protected.