I think you'll have to confront her on this again and explain to her that she is showing you a lack of respect. Regardless of whether you are wrong or right in her eyes, she needs to follow your directions for your children. If she still continues to disrespect you, then you really should not allow her the opportunity to feed your children. That might mean staying away or taking them with you when you are over there and have to go out. I suspect just 2 or 3 times of doing that, and letting her know why you are not letting her keep the children, will nip it in the bud.

I wish I had known about lc when my daughter was young. I think it's great that you are giving them this head start on a healthy way of eating. Good luck!