Yeah, go with the MCHN angle only a bit thicker with out telling a lie. Especially with the allergies.
Also say when you speak to her that you are not being disreapectful but you need to consider your DD's well being before anything.
Also ask her that if she knows she is feeding her the wrong foods why is she doing it? If she cannot come up with a good answer then tell her that until you can trust her to feed Olive as you have asked you will not be leaving her in her care. it's then out in the open and it's also up to her how she reacts.
All the best.