A child who says "I have been abused/raped/touched, etc is the first witness. The second witness may very well be the medical report that the police would pursue if and when they were contacted. The second witness then becomes the physical evidence that the doctor finds. Like ripped anal tissue, vaginal tears, etc. Second witness is forensic evidence that is discovered if and when the child is taken to the police/doctor.
You are trying to pass off your personal beliefs and personal definitions of what is evidience, plaintiff or witness as blanket International requirements that should cover every jurisdiction, fact is they don't, you are confused.
In the US, it doesn't take much to have someone investigated for child abuse but you can't go around accusing people of charges. If such accusations turn out to be false, the person making false accusation faces defamation proceedings besides ruining the life and reputation of an innocent person. Who would ever trust someone with a child who was accused of child abuse even if he later was found not guilty in Court? Some people don't care.