Many of the characters in ASoIaF have some sort prophecy attatched to them, it just so happens that one of these people is Cersei Lannister, one of the series most prominent and persistent antagonists.
Now until Winds of Winter comes out in 2076 or whenever George gets done with it, we can’t be precisely sure how Cersei’s stroll down naked lane has effected her psychologically. Maybe she’ll be a changed person? Maybe it’ll make her even more paranoid and prone to poor decision making? Who knows.
What we do “know” is that when she was a kid a fortune teller from Essos told her this:
“Aye. Queen you shall be… until there comes another, younger and more beautiful, to cast you down and take all that you hold dear.”
Cersei being Cersei assumes that “younger Queen” is good ole Margery Tyrell, and you can’t really blame her for that. Except, as many a fan has pointed out, Dany is a younger and more beautiful queen, and she’s also considerably more dangerous than the Tyrell queen.
What does Cersei hold dear? Her power and (her one redeeming quality) her children. Cersei loves her kids, as most mothers do, no matter how wretched and megalomanical the mother may be.
So let’s assume Dany is the younger Queen, It’s pretty easy to see how she could take Cersei’s power (perhaps some form of fire, and some form of giant flying lizard may be involved) but if Dany is going to take ALL that Cersei hold’s dear…then Myrcella and Tommen may be in trouble.
We also know that Cersei will outlive her kids, and she’s already buried one of them. So if the younger Queen is going to take what Cersei holds dear, and her kids are one of those things, and her kids are going to die before her…is Dany going to bring about the deaths of Tommen and Myrcella?
Disclaimer: I’m not one of those people who says Dany is mad, not at this point. But I will say that like all the Targs she has a higher chance than the average Westerosi of one say drinking a bottle of wild-fire to turn into a dragon, or just burning people to get her rocks off.
The big problem, at this point, is that it’s really really hard to see Dany slaughtering two, pretty nice, kids in order to get the throne. She holds a severe grudge against King Robert for doing just that (even though it was Gregor and Tywin, but whatever).
If Dany is going to be the one who brings down Cersei then, if the prophecy is accurate (and we’ve already seen that Maggy was pretty on the money) it will require her to do the exact same thing Robert had to do to win the throne. Eliminate any other claimant.
It’d be an interesting development and a good piece of dramatic irony. What would it mean for Dany’s character? Could it be a catalyst for her going mad? Would she need to be mad already? Again these are questions I can’t answer and there is also the fact that there have been other characters suggested other than Dany and Margery who could be the “younger more beautiful” queen. Sansa for instance, but I can’t see her killing children either.