Even if the children are underfed, that problem will not be solved by an undermining grandmother constantly trying to slip them treats.
My 5 year old has been underweight from birth. He probably always will be, if my FIL is anything to go by. Filling him up with sugar and icecream solves nothing at all. I give him nuts, and cheese and fruit for snacks. Many days he eats more than I do, and he still doesn't gain weight.
It's not a relative's place to go against all the parenting decisions. If OP thinks the kids don't get enough food, she can try to speak to her daughter, but she has no business teaching the children to lie to their mother. That is not only unkind and wrong,it's incredibly dangerous.
If I was OP's daughter, and I found out about the deceit and lying, OP would never be alone with my kids again.