Is it illegal to use someone else's Facebook ID and password to gain access to a minor's private Facebook page.

When you ask a question by saying "is it illegal," the answer isn't likely to be useful. In our free society (the Commonwealth of Pennsylvania in the United States of America) everything you do is "legal" unless there's some law or regulation or right to the contrary. But there's an intricate patchwork of ever-changing laws, regulations and rights that nobody -- not even lawyers -- can fully keep track of. And each of them has its own consequences and modes of enforcement. Most of the time, when we think of something as "illegal," we consider whether someone who violates that law could be criminally prosecuted. If the law is federal, that would be enforced by a federal prosecutor. If a state criminal law is violated, then it would be enforced by prosecution in a state court. If the activity violates both a federal law and a state law, then the crimes might be prosecuted by either or both prosecutors. And the result might be a criminal fine, incarceration, or sometimes another mode of punishment. Some actions aren't criminal but trespass on someone else's rights or violate a contractual obligation. In that event, the knowing act should be considered "illegaL" but not criminal. The owner of the rights would have to enforce that right by a civil lawsuit against the actor. Other actions might violate a federal regulation (e.g. by the Federal Trade Commission) or a local ordinance, and would be enforced against the actor by the pertinent agency. Some legal rights may be enforced by an "exclusionary rule" applied in court to prevent improperly-obtained evidence from being introduced in the court proceeding. I wonder if you may be asking whether there would be an applicable exclusionary rule in the circumstances you outline. Usually the answer to such questions are "it depends," and for actionable legal advice, one should hire counsel in the pertinent jurisdiction and confidentially detail ALL the facts so that the various aspects will be fully considered.

I see a number of hurdles here. There are laws against misrepresentation, but those laws protect the minor, not other persons. Moreover, people fake access to Facebook regularly, and part of the danger on FB is that individuals don't take precautions against friending everyone. So, a strict showing of reliance would be critical. Another problem here has to do with posting photographs. Photographs, even those posted to private pages on FB, may be found by Internet search engines, regardless of whether the individual has permission to be on a FB page. Finally, anything posted to FB may be used in legal proceedings as evidence in PA; the PA courts have been pretty clear that anything posted to FB, even when set to private, may be admissible in court proceedings. Finally, if you have a judge who thinks its fine to monitor kids online (which many do), they will not be swayed by a child's "right to privacy". So, this is probably going to result in a disappointing outcome, given the facts you've provided. Best of luck, though.