I think that Alan rickman would make a great Aro!
They need someone evil looking that can pretend to be nice. Also I always imagined them around that age. That's so weird that Jane and Alec were supposed to be 12 I thought they were adults but just really petite, I thought michelle trachtenberg would be good as she did evil well in gossip girl!
I bet the little girl that played lyra in the golden compass would be good! Dakota Blue something, she could do evil! I reckon they'll pick newcomers or small names for these parts though, they don't do enough in the 2nd book to get big names