Child Welfare Agency
Many countries have specific Child Welfare Agency or department, who deal with the case of children neglect and abuse. Some countries have broader departments of social welfare, human welfare or community welfare. No matter by what name almost all countries do have social welfare departments. It is better to directly report them the case of child neglect or abuse you know. You can find their contact details or help a person find contact details online and the person to contact to. Timing is important factor and you should not delay the reporting.
Child Help Hotlines
Some countries have purposeful children help hotlines and their contact numbers are publicized on a regular basis. It is better to know or keep the contact details of such helpline and help a family in reporting a case of child abuse. If you spot a child is being neglected or abused within or around the family, immediately report the case on the hotline. You can expect to be asked about some details about you, but it is to know the authenticity of a person who reports the case, not to scare you away. It is better to be able to provide some basic answers about your relation to the child and how you have come to be aware of the case.
N0n-Governmental Organizations for Children Welfare
Some countries, especially in third world do not have a recognized children welfare agency or a helpline, or they have but are almost non-functional. In that case, it is more likely that an NGO will be working for the welfare of children. In some countries there are NGOs for rehabilitation of lost and abused children. You can report them in case you do not get help from a government department to timely protect the child from neglect and abuse. Many organisations have online reporting systems, you can fill-in a form and report the case.
In case that you have tried everywhere else without getting much help or you do not have contact details of another agency, you can directly go to nearby police station and report the case of child neglect and abuse. Also, if you know that a case of child neglect and abuse is in more critical stage or being commented at a larger level, immediately report the case to police or to any other law enforcement agency. This will help save life of the child.