Who do I report an court appointed childs attorney for not investigating an issue

It might depend on whether you are in Superior Court for either a Dissolution of Marriage or Complaint to Establish Paternity, or Dependency Court. If Superior Court, I would suggest bringing the issue before the judicial officer to explain your concern about both the issue and minor's counsel's unwillingness to investigate it. I would caution you however that the likely reason that it was not investigated is that the child's attorney must have thought that the allegation was either not sufficiently substantiated or not believable, so be sincere in your concerns before you raise them with the judicial officer, and do not use them to get a 'heads-up' in whatever custody battle is raging.
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If you are in dependency court, I think I might discuss the issue with the social worker on the case, and then with the judicial officer if necessary.

Bring these concerns to the judge overseeing the case.