The need to protect your child from harm is a very serious thing. Don't trust people who don't respect the ideals that you have concerning taking care of your child. How strong is your conviction? Can you sternly tell her how serious you are about not wanting to feed it what you don't want it to have? If so, provide its food and tell her no other food. If you find out she is violating your wishes then she is disrespecting you and the very health that you are trying to provide for your child and that is a very serious offense. That is a beautiful thing to care for a child that well and if someone can't respect that then you should do something about it. Don't play games with it, there doesn't necessarily have to be too much explaining to her, that is your child. Protect it. If she doesn't promise or if you don't trust her then yes tell her that it means too much to you to allow your child to be with someone who doesn't respect your protective wishes. Visiting may have to wait until the child is old enough to control its own consumption without devious interference of others. Hey a soft approach may work but it may not.