What are the children rights when a family member moved our Dad to a new state and closed the bank accounts?

You need to contact a lawyer.

Just because someone held dad's power of attorney doesn't preclude the nephew from taking dad to his banks and closing out his accounts on nephew's recommendation. That's because a POA doesn't mean that ONLY the POA can transact business. Your dad could transact his OWN business unless someone had obtained guardianship through the courts. Obviously that wasn't done.

Forget it. Chalk it up to "His children should have been paying more attention."

Too much information missing for us to guess. Was Dad in his right mind when he moved? Did he go willingly? Did he have dementia? The nephew could not establish himself as the POA -- Dad had to sign that. Was he competent to do so? You may or may not have a legal case. See a lawyer.

"even the one my father left for me." Obviously your cousin convinced Dad that he cared more than you did. Too late now.

Did you act on the matter when your father was "stolen", or after he passed away and you realized the accounts were redirected.
How long did cousin keep (caregiver for ) dad? Was dad well taken care of? Did you investigate that? Why did dad leave? Was dad mentally competent, although he had dementia?

If someone had "taken" my dad, they would have had to deal with me.

family58, just curious why the nephew felt he had to become your Dad's caregiver?