Kyoko sees the world through a mercenary lens (at the moment), so the best way to get to her would probably be to suggest an alliance against Walpurgisnacht. That way, cleansing her gem/grief seeds will not be seen as hopelessly doomed optimism on one hand or part of an underhanded plot on the other, but rather as the pragmatic support of an ally.
On the other hand, if we don't want to tell her about Walpurgisnacht yet (e.g. if we want to tell Mami first), we could try to give her the whole "give us 3 full grief seeds, and we'll give you back 2 empty grief seeds" option.
On the other hand, if we don't want to tell her about Walpurgisnacht yet (e.g. if we want to tell Mami first), we could try to give her the whole "give us 3 full grief seeds, and we'll give you back 2 empty grief seeds" option.