<raises hand> I'm a grandma who lets the grandkid push the cart....but I get JUST as irritated as you do with Moms or Grandmas who do so without much supervision. My littlest grandkids LOVE to go to the grocery store with me, and sometimes as an "adventure" for them I'll take one of them. It's a big deal. We look for the kiddie carts (the scaled down version of the big one) and they push that around. Occasionally there's no small ones available, so we have to use the big one, and in that case we'll do it together, making sure to stay to the side of the aisle because we're slow. They typically get tired pretty quickly of pushing the big cart because it's hard to manage, so then it's just a matter of "helping" them make good food choices and keeping them away from the deli section where they give out free cookies to the kids. They're allowed one, and they don't get to circle back for more.
