For this kind, a quick death is a mercy. What we call hope torture should be ideal. Let the water cool down a bit, allowing the yukkuris to take it easy again (and probably think something like "Stupid mister human is scared of great Reimu and made mister water cool down again") and then resume the boiling. Repeat from step 1 until the yukkuris get the lesson or just die.
Warui, I believe the children hiding in the Reimu's mouth are eating her from the inside out. It seems the font being used to say "munch munch" is not the mothers, and belongs to the Marisa saying "the center is piping hot".