After I have migrating the domain name and the content files to a new hosting provider, I then try to open the child pages of WordPress and there I see the 404 error.
This error is very general in nature and you will not get a clue of what is the actual error. Because I have moved the site to a new hosting so I kind of know what is the issue, and this happen when you set the site to use a permanent link as custom structure.
Here is how to fix it:
1. Login into WordPress administration.
2. Click on Settings > Permanlinks.
3. Now click on Default radio box under Common Settings, and click on Save Changes button at the bottom.
4. Next click on Custom Structure and set the URL structure again.
5. Finally click on the Save Changes button again.
Doing the above steps above will miraculously fix the problem.