"Letting them die" is something Riker makes fun of, with his "fate" comments. Nobody seems seriously concerned about that aspect of the discussion.
Riker says that letting them die is an option they should be considering and both that and his fate comment are completely in earnest.
the debate is entirely because Picard feels that violation of the Prime Directive "however well intentioned" is wrong. Its clear that the girls pleas do sway him but he is still somewhat unsure. After all if he was sure he would not call a conference discussing the prime directive in the first place.