I don't think there is any easy way to tell grandma to back off. You either listen, ignore, or tell them to back off . Just wait until she starts telling you things like "that childs feet won't be right if you don't put shoes on him." (my husband and I actually heard this one about our first son when he was six months old).

I would like to tell you its a generational things, but even ladies my age give me "advice" about what is and isn't right for MY child. I've learned to smile and say something like "you know, I think I've heard something like that before." Acknowledging them usually gets them to back away. Notice I didn't say I followed their advice though. What they don't know won't hurt them.