My child, 3.5 has a grandparent (my PIL) who, when it's time to say goodbye, asks for a kiss and kisses my child on the lips. It makes me really uncomfortable for some reason and I feel it's inappropriate. I want it to stop before my child feels uncomfortable too as they're getting to the age where they soon might. I also have a 6 month old and I don't want this habit to start with them too. What can I say that's low-key enough not to cause upset but firm enough so that they know I'm not happy with it and that it never starts with my baby?