Flexbox children with display flex

I have a question regarding (immediate) children of a parent with display:
flex, i.e. a flex container. Can a child or flex-item also have a display
of flex associated with it, thereby becoming a flex-container itself?
I was unable to find the answer here: http://www.w3.org/TR/css3-flexbox/
What is the spec's take on this?

I made a basic demo here: http://jsfiddle.net/qeaayghn/
The .section-top div is a flex-container and each div with the box class is
a flex item. I've made the last box a flex-container again, so I can center
the image inside its child with class box-child.

Of course, the image could be centered in many other ways, but I'd like to
know whether it violates the spec, in the first place.

Regards,
Karen