Easy, she was probably half his age, and either leading him, or holding on to him - it isn't to difficult to visualize at any rate - if you are telling someone not to look back, and you are walking hand in hand with them, it is quite possible to see them be turned into a pillar of salt, without having to look back - looking to the right is not looking back.
Either way, it's a moral lesson - the point is not to look back, no matter how frightened, if God has made it clear there is nothing to fear if you follow him. Whether it was seen or not is not a very interesting question, in my honest opinion - rather, whether, if it indeed was the end of the world, would Lot's daughters actions be justifiable, that's a better question.