Are cursed children vulnerable to varanium? In Black Bullet all cursed children carry the Gastrea virus, basically making them Human/Gastrea hybrids. It is shown that Varanium harm the Gastrea.
Now, the question is are cursed children vulnerable to Varanium, or does their human side grant them immunity?
So after reading the light novel, I can now answer my own question. Cursed Children are in fact vulnerable to Varanium.


  "But something seems strange...," said Enju. "Ever since coming here, I've been feeling excited for some reason." Enju opened and closed her hands curiously. Of course you would, agreed Rentaro silently. The Varanium that Gastrea so hated also had an effect on Enju and the other girls, who were all infected with small amounts of the virus. Most Initiators, when they went outside the Monoliths, felt temporarily better, or even high. Their wounds also healed faster.


So clearly while inside the Monoliths the Cursed Children feel their effect. While it is clearly not deadly to them it does have an effect on them. It is also possible for Gastrea to survive inside of the Monoliths, though it seems impossible for them to pass between them. The difference is probably that Cursed Children only carry a small amount of the Gastrea virus inside them while a regular Gastrea carries a large amount.

The following passage shows that Varanium bullets have the same effect on Cursed Children as they do on Gastrea


  Enju's wound showed no sign of healing and was still oozing blood. Varanium bullets prevented a Gastrea from regenerating after being wounded, and it was no different for Initiators who were able to regenerate thanks to the Gastrea virus. Facing Varanium weapons, the was as vulnerable as a regular human being.