I just traveled from SA to Thailand with our two Thai children . We went to Thai embassy in South Africa got and Thai consent letter with fathers Thai ID and my ID attached also went to local police station for permission letter to leave South Africa with children father had to sign. Also attached was children's birth certificates fathers ID my ID my passport and visa . But inquire with immigration at the airport in India what documents you will need to leave with your child . I did this trip on the 20th April and they asked for these documents when I left and when arrived in Bangkok.