First of all, you can report the case. In Hong Kong, parents can report the case to the Hong Kong police and provide relevant information and evidence about the disappearance of their children. The police will investigate and take necessary actions.
If you suspect that your child has been trafficked or illegally transferred across the border, you can contact the relevant mainland authorities and the Hong Kong police and cooperate with them in investigation and assistance.
In addition, you can seek legal aid. Hong Kong has an independent judicial system. You can consult a lawyer to understand the legal measures and procedures that can be taken in this case.
What needs to be explained is that I can't understand the specific situation. The above answers are only general suggestions, and the specific situation needs to be dealt with in combination with the laws and regulations of your country and region. I suggest you communicate and consult with relevant departments and legal professionals in time.