I'm wonder if Edric is important enough to cast. The point of his character was another acknowledged bastard son of Robert that provides more evidence to the claim that Joffrey is not Robert's son, and the whole thing about using Kings Blood in fire magic. Unless they go there with the blood sacrifices, which they haven't so far, I would guess Edric ceases to be relevant. Other minor characters, such as Jeyne Poole were reduced to extras on the TV series, I think Edric could be treated the same. If they're going to cast anyone around Shireen, I would think it will be Patchface.
That's a good point, especially because the scene with Melisandre and Davos in episode 2.4 was done differently in ACoK. In the book, it's to kill the guy holding Storm's End (and Edric Storm) with a shadow-baby, not Renly. The fact that they switched the scenes and didn't even bother explaining the context makes me think that Edric Storm won't be important enough to add to the show. They'll probably just find another way to show that Melisandre is crazy. Maybe she will want to sacrifice Shireen instead?