I'm gonna be controversial and say NTA, because people have to wear things they don't necessarily want to but context makes refusing awkward. Like would you let your kids dress up in jeans and a tee-shirt for an important event like a wedding or church or eating out at a nice restaurant? Sure it's outdated, but unless you're visiting on a weekly basis, this is just another "upscale" activity, or at least that's one way of looking at it. Kids can express themselves however they want, and let the girls pick out a couple of dresses they don't hate. I'm tired of this insane idea that you're breaking their spirits and abusing them by requesting they dress nicely (wear a dress) to their grandparents. They can hack it for a few hours or they can stay home I guess.