Blessings for a child

Make a prayer to Santhana Gopala by chanting the following mantra 108 times a day for 108 days after bath.

Santhana Gopala Mantra

Om Devaki Sudha Govinda
Vasudeva Jagath Pathe
Dehimey Thanayam
Krishna Thwamaham
Saranam
Kadhahaa Deva Deva
Jagannatha
Gothra Vridhi Karap Prabho
Dehimey Thanayam Sheegram
Ayushmandham Yashashreenam !

Note:

Nagaraja is the devata for Kudumba Vrithi. Perform Nagaraja pooja in your home by some learned priests.
Also visit Rameswaram, Devipattinam and the navagraha temples around Kumbakonam with your saha dharmini and make prayers to the devatas.

Mantra for a Child birth

This mantra may be recited for getting a son, or for childless couples, who want a child.

Mantra:

Devaki-sut Govinda Vasudeva Jagatpate
Dehi me tanayam Krishna Tvaamaham sharanam gataha

Meaning:

O Son of Devaki and Vasudeva, the Lord of the Universe
O Krishna! give me a son; I take refuge in you