can adult children sign property contract on behalf of alz father living with them? does their name need to be on contract too

The short answer is no.  The father is buying (selling?) this property; the kids are buying (selling?)nothing.  Assuming the power of attorney is valid (you need to see the check out the power of attorney and you might want to get the kids to agree to indemnify you in case anyone challenges it at a later date), the father has given his children authority to enter into the contract on his behalf; they are merely his agents.  The contract can be signed "Father Doe by his attorney in fact, Child Doe". That being said, if you're not comfortable not having the kids' names on the contract, nobody can force you to agree.