dear johny as per my experience i brought my 68 days old son on arrival visa, they will issue 1 month visa for your baby.you should carry original birth certificate. and father and Mother should be present on arrival.
once they arrived in qatar you should go to garraffa main immigration (better you go to morning shift) for RP stamping
you just carry following doc.
PP and RP copy for mother and father
filed application copy
photo of the baby
original birth certificate (no need attestation)
Note: they will issue only 1 one year Residence permit for your baby and 400 Qr for cost of the RP