prahlada is not in santa rasa. he is in dasya rasa. he is a bhakta of narasimha (visnu).
The symptoms of Prahlada's bhakti are not in line with the symptoms of Shanta Rasa given in Bhakti Rasamrita Sindhu - love for solitude, love for Yoga, love for the Brahman and Paramatma aspects etc. nor is his name, as far as I can see, mentioned by the Gosvamis as Shanta bhakta. Prahlada was an ecstatic bhakta of the Personality of Godhead Narasingha.
CC Madhya 19. 218
shantera svabhava krishne mamata-gandha-hina
param-brahma paramatm-jnana pravina
TRANSLATION
It is the nature of shanta-rasa that not even the smallest intimacy exists. Rather, knowledge of impersonal Brahman and localized Paramatma is prominent."
PURPORT
Because of an impersonal impression of the Supreme Personality of Godhead, a devotee in the shanta-rasa relationship worships the impersonal Brahman or localized aspect of the Absolute Truth (Paramatma). He does not develop a personal relationship with the Supreme Personality of Godhead, Sri Krishna."
Does this sound like Prahlad ji to you? Study his carita in the Bhagavata, 7th Skandha! Then you can see the ecstatic symptoms he displayed.
Thank you Advaita Dasa
