It's an interesting idea and I'm quite a fan of random unpleasant nastiness.
But I think murder is too great a leap and you haven't the build up to justify something that horrid.#
So my suggestion.
She has one spoonful fine, second spoonful she digs up a massive turd dad has obviously shit in her cereal.
Sink scene
She dives her hand and pulls out her dead kitten that mum had obviously drowned earlier.