I'm super pale and burn easily (my child is the color of milk chocolate, so you're unlikely to see a visible burn on him), but I don't turn red while I'm burning. Instead, I stay nice and pale until I get out the sun and then presto -- it's like a magic trick.
So, if you saw me bright red at WDW, it would mean that I forgot the sunblock yesterday, not today, and offering it to me today wouldn't help. After all, since I'm already in pain the chance that I forgot today (e.g. that the pain didn't remind me) is minimal to nonexistant.
But like I said, if you said it nicely (e.g. Would you like to borrow some sunblock? Rather than "How dare you let your child get that red!" I wouldn't mind).
On the other hand the other day I went for a walk, and had my son ride ahead on his bike. About 1/4 of a mile in, my son stopped on a park bench to wait for us. Some other biker saw him, saw that he didn't have water (because it was in my backpack, and I was about 2 minutes behind him at this point walking) and felt the need to scream at me "He needs water! Why wouldn't you carry water!". If you spoke to me like that, then I'd not be amused.