They allow sexualized imagery of children at DeviantArt? Find that hard to believe.

Oh they do. WHen you report it you get back "can you prove the figure is a child". Me and several other artists have been fighting this for awhile. Even when you link them to the child model, in child clothing, etc they still keep with the "can you prove its a child". Its not pleasant.

I have taken to reporting images to authorities because DA simply wont do anything.

artists like KassidiKeys have even left due to the issue.

If you want to battle DA on this then you'll need to make the company that hosts DA aware of whats going on.  You'll need to provide the link showing the child images.  Do this in email because it provides a virtual paper trail.