M.P.
I am so sorry you have to go through this. It's definitely not fair. The question I wonder is... what if your child is overweight - is grandma going to love her less? Think less of her as a person. It's really important to instill in people that life is not all about looks. I am a moderately thick build and my husband is about the same - we are not small people, but we aren't obese either. Our children love to eat - almost anything. They love their chips as well as their broccoli. They are willing to try new things. But they also know that if they overeat, it will be harder to be energetic, to keep up with their friends, etc. I am lucky that they are both active and fit. They play sports, but they will never be petite. They do get teased about it by people who are thin. I explain to my children that body size and dynamics is complicated and that it's just as important to take care of your brain and your feelings as it is to take care of your bodies. If you can somehow just "discount" grandma in a way that your children understand without being disrespectful, then I encourage you to do that. If you engage in an upfront struggle, then the kids will feel that it is a big deal and it is polarizing. I'd probably say something like, "Oh, Grandma's being silly again! She'd probably like you to stay small forever, but we know that's not going to happen - you'll grow the way you're supposed to grow based on genetics, food choices, and exercise." There are also great picture books about bodies for kids and that is an objective way that they can get real information at their own levels without depending on mom or grandma who they probably love and don't want to see disagreeing. I would say to my mother-in-law, (This actually happened with me in terms of weight, intelligence, and general personality) "Mom-in-law, I know you have your opinions and we need to appreciate what they are - opinions. But I don't want you sharing your opinions with our children. We adults in the family need to have a united front, so if you want to say something of a personal nature somehow judging them, tell me privately and we can discuss it. Do not discuss it with the children. It gets too confusing for them." And then if and when she does come to you instead of going to the kids with her concerns, then praise her and thank her and respect the deal you made and listen to her.