Our child is a minor US citizen and will travel to Puerto Rico with a group not including us, her parents. We understand that, to make this trip without parents, she must carry an official photo ID with her. Can her expired US passport serve as the ID for this purpose?
Assuming there is an adult accompanying the group, the TSA does not require any ID. The TSA says:
TSA does not require children under 18 to provide identification when traveling with a companion within the United States. Contact the airline for questions regarding specific ID requirements for travelers under 18.
You should therefore ask the airline what identification they will accept.
The Transportation Security Administration does not allow you to use an expired passport for identification, as all federal or state-issued photo IDs must be valid and current. However, if you're traveling with an expired passport, you may be able to show TSA alternative forms of ID to verify your identity.