J.A.
As much as it hurts to hear, this isn't Costco's fault! Granted, she is 6 and not mature enough to look for the warnings- but we are the parents and it is our job to warn of the dangers that are present. If she had walked across a street without looking and a car driving under the speed limit hit her and knocked her to the floor, would you sue the driver because your daughter wasn't obeying traffic safety rules? (This is the exact senario of what happened years ago with my mom as the driver. The kid hit the side of her van trying to run across the street. It was detirmined at the time of the accident that it was the childs fault and that my Mom had zero blame, but the girls family tried to sue anyway.)
I thik this falls under the "McDonalds Million dollar Coffe Law Suit". Yes, perhaps you could win in the basis that a 6 year old can not comprehend the risk of doing such a thing. However- common (adult) sense says "Hey, this is used to keep the food I'm buying hot. Touching it is probably not a great idea!" If fighting for something that should be cammon sense to avoid is up your alley, then go for it. I personally would let sleeping dogs lie.
Years ago I believe that I DID have grounds for a law suit againt Carl's Jr, because I got a 2nd Degree burn just above my lip while biting into a crispy chicken sandwich. The sadwich had an oil pocket under the fried skin, and when I took a bite, the oil burst all over my upper lip, causing a blister the size of a quarter in less than 2 minutes. I thought about the Million Dollar Coffee, and that I could likely win, especially since I had no reason to believe there was scapding hot oil in the sandwich... but I also thought about the fact that Carl's Jr didn't intentionally allow it to happen either. My brother wanted me to contact them atleast to get a couple of free meal vouchers, but I didn't even do that.
I whole heartedly believe that the actions you take tward others will come back 10 fold. If I choose to extend grace to someone, 10 others will extend grace to me. If I choose to engage in pety confrontation with another, 10 others will engage with me. (I don't find your concern for your daughter pety!)
Before you take any action, consider if it is TRULY on behalf of the safety of "future victums" or out of anger that you didn't forsee the outcome before it happened. {{{Hugs}}} to both you and your daughter!