Can children of disabled parent talk to doctors in a hospital when someone with a durable power of attorney in Nevada tells them not to?

What is your compelling reason for wanting to do an end-run around your parent's wish and the authority of the DPoA?

As Isthisrealyreal says, anyone can allow a doctor by their express permission to share information with you. But if a person is unable to grant that information you are up against a wall. Certain ANYONE can speak to a doctor. The doctor is often hindered by laws to give any information back if you see what I mean.
Realyreal is also correct in saying that if you are THERE the tendency, with caring family, is for staff to kind of off the cuff share stuff. That is dangerous for them, so they are sometimes unwilling.
Is the POA in another state informed and participating but unwilling to share info with the rest of the family? Is this a family problem? Hope not. Try your very hardest to get along. Even if currently this person will not share with YOU, you are there, and you can share with this person, willingly and openly what you are seeing; this may open communication. Try your very best for honest,gentle, open communication at this time. Good luck.

Can the patient sign a HIPAA release form?

MPOA/DPOA has the ability to control everything if the person in question is declared incompetent or is unconscious.

Are you at the hospital and the POA is in another state?

I didn't have POA for my dad but I was at the hospital all day and every other night, so the doctors and nurses just shared everything with me.

They can probably talk to the doctors but none of the doctors will do more than listen, they won't answer questions or disclose information without the consent of the POA (or the patient, if they are competent).