I would leave food for her and the kids. Like for example: here are some apples and lunch for the kids, and I made this for you, I also left you some snacks.
Put their food out and have it plan. Tell her that you don't want the kids eating nothing else but the stuff you left for them. Tell her that everything else you have in the fridge is for the next few weeks (which you already planned).
If you leave yummy things for her, I highly doubt she will go for your calorie saving snacks.
Ps: I always thought it was rude to eat other people's food, but that is just me