As a new parent I found it impossible to say ‘can you not do that’ although I told everyone not to do it, when they did I froze and felt sick and said nothing. I think you should get your partner to tell her, just say I know she’s so so cute and you want to kiss her, but Becuase she’s a newborn we’re not letting anyone kiss her until she’s a bit bigger with a stronger immune system xxx
I have explicitly said to my mother in law that if she kisses my baby she doesn't get to see my baby. She also knows I'm dead serious. Luckily, she's been rather respectful so far. But I have no qualms about removing access if she crosses me. My baby, my rules.
I wouldn’t say anything now because it’s passed, but next time she comes I’d say something like ‘would you please wash your hands before holding X, oh and please don’t kiss him’ Quite matter of fact but not unfriendly, and said with a smile ofc. ☺️
Personally I'd have snatched my baby back and tell her I explicitly said no kissing. I did just that with my sister in law!! I feel your anger!