Is it necessary to have birth certificate (Kids) for visit visa

If the child is attached to his mother passport then it is not required. If the child have separate passport and his fathers name is matching yours also not required. It is required only if nothing of the above is valid, the relationship has to be proven for Qatari Authorities by any approved document - attesting the same is NOT REQUIRED.

Sure, to sponsor child, you need their birth certi and to sponsor wife you need marraige certi.Both certi should attasted by local authority, concerning embassy and mini of foreign affairs,(near musiam.old salata)

I think you need birth certificate of kids, but attestation of the copy has to be done here first at your country Embassy and then in foreign affairs dept, you may need to show the original at your Embassy.