My MIL wants to give my four month old chocolate!?

So easter is coming up and my In laws have a seafood lunch on good Friday. My MIL keeps saying to my four month old daughter "Grandma might even melt some chocolate between her fingers and let you have a taste." I do not want her to do this! I am almost certain this could make her sick! My MIL is a very sensitive person and whenever she is told no she takes it very personally. I have tried be extra nice and careful when saying no to her in the past but she always gets so offended. I actually don't think she has a clue as some of the things she has tried to do with my daughter are just ridiculous. I cannot tell my partner to tell her no as she does not take him seriously and does not respect him as the parent of her grandchild. How can I say no to her without her giving me that teary, sad look again?