can I apply China tourist visa for my children in HK?

I think so. Even if you can't get the visa in Hk yourselves, you may turn to China Travel Service Hk Limited for help whose contact information is telephone: (852) 2315 7171, email: ctshk.com|enquiry.

Why would children be denied in getting a visa to visit their parents?
My children are age 9 and 11. Why would children be denied in getting a visa to visit their parents? Can they re apply" at anytyime after the denial? and in the same embassy in Manila as we don't have time to do it in HK?

No body knows the reason of the visa rejection to your children, since China embasy has the right to refuse any visa application without giving any explanation. they can re-subnit their visa application again to the same embassy in Manila any time. I advise they contact the embassy to know about the exact documents for the application first before submitting the re-application.

Thank you! Do we have to tell the embassy we got refuse from previous application?

No need to to so, just submit the application again.