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.
Varaniums are used as bullets. So far cursed children aren't shown to be bullet proof, albeit their superior physical strength. So of course cursed children would still get hurt if they are shot using it.
As for whether the damage would escalate due to the gastrea virus in them or not, it can't be said for sure for there hasn't been any scene that shows a living cursed children shot by it yet. I said living since when Rentarou shot the type Dolphin cursed children, Kayo, she was already dying from the damages she received and the amount of gastrea virus in her has exceeded the critical value. The bullet used by Rentarou there would definitely be a varanium bullet as they were in a battle against gastrea and normal bullet don't work against gastrea, which would means that there is no reason for him to bring normal bullet with him.
As shown in the scene where Rentarou received Enju's exam report, the critical value or the limit of gastrea virus inside a cursed children's body before they turned into gastrea is 50%. Thus, at the time Kayo was shot, she is already more of a gastrea than a human. Because of that, if Kayo received a Super Effective damage from being shot, it would be arguably because she is already more gastrea than human.
Just touching is okay, as shown when Enju touched Rentarou's arm which is made of Varanium, although arguably she might be okay since it wasn't a direct contact due to Rentarou's skin.
The other use of Varanium is as monolith to prevent gastrea from entering the populated area. As far as I recall, there was no scene where Enju or any other cursed children touched the monolith.