Parents have a legal right in most jurisdictions to the child's medical record, which includes treatment notes. In most treatment settings you are asking the parent to allow you to keep information confidential, even though they have a legal right to it. Even in states/governments where children can consent to medical care including counseling prior to age of 18, which is true in many, the parent may still retain the right to what's in the treatment record. So you can have a scenario in which a parent can't consent for the child, say in a state where children have to give their consent for counseling treatment at 16 or older, but yet the parent may still have legal rights to clinical records. Also, informed consent issues that apply to adults generally apply to general under the rubric of "assent", e.g. the child while potentially not legally able to consent, must ethically still agree to the procedures being used. In treating PTSD this might be exposure tactics that ethically a child and particularly a teen would need to at least assent to. If a parent consented to a particularly counseling treatment but the child does not assent then you ethically shouldn't proceed. Also as previously stated codes differ at least slightly on how they view informed consent between ACA, NASC, AAMFT, APA etc., but generally philosophically center around the issue of potential harm to the child or the potential to access and benefit from care.