Well these are well known facts to any kid who is beyond first experience with styrofoam cups, heck apart from appalling chemistry being leaked into drink, I do actually prefer cups that keep temperature longer when ordering drinks. That's the whole purpose if styrofoam here. Heck, there is whole business around thermal mugs for exactly this purpose. Same would be valid for standard tea - you start with cca 100C and its actually how it should be. If you don't like it hot, order something else or go someplace else.
If we would be talking about case of little kid not knowing this type of cup burning themselves, even way less than she did, then that's a completely different story. Or was she also somehow mentally impaired to not understand hot=bad in groin? But as facts are presented, people do themselves harm with stuff that is well known and expected to cause harm.
If I let chainsaw in my lap and accidentally start it somehow via normal way it should be started, well that would be also life-changing injury, and result of my utter stupidity and nothing else. I do get I could sue the company making it, and if in US I can actually win but that's ridiculous.
Sorry, as much as I hate corporations including McDonald for many, many of their transgressions, this one is properly ridiculous even with all the facts stated and all the time passed. There is no end to people getting themselves injured or killed every single effin' day, even right now, doing stupid things with dangerous stuff. Some internet sites are just endless daily streams of records of that, and its pretty damn crazy. So instead of lesson being 'be careful with dangerous stuff, the dangerous part is serious', we bend reality backwards so hot coffee is not hot, just warm, because somebody will put it in their laps, or throw it on another person, or will pour it over their face.
Instead of saying ie - McDonald, please present 2 types of cups, one for people like me, call it 'hot that lasts' and charge extra 20c for it as an added value, and another 'warm coffee that won't burn ya even if you shower in it'. I really don't see actual moral lesson for McDonald in this case. A little bit of personal responsibility expected from citizens would be much, much better lesson for society as a whole.