I am trying to find out how to get information on my child who is in her mothers custody.

You may not be able to unless it is part of the child custody agreement. Unless you have joint legal custody or unless the child custody agreement says she has to tell you, while I would see a moral duty to tell you, there is not a legal one.

If you have court papers giving you that right, you can send a copy to the doctor with request for records.

But you may have to get an attorney anyway.