Cool! So here is a more involved pen. (Managed it on the sly, very covert!)
https://codepen.io/danielhoff/pen/vbyrZd
I've created a grid with 36 cubes, selected the .box class and shuffled the array. It's the next bit where i get a bit confused, so to stagger I would have thought the selector would have to be:
boxes(the array) .front(the child)
to then stagger those specific elements.
the only other way I think is to put each child into an individual array but that could get messy with the randomising.
I also think maybe Three.js might be the way to go as atleast then the cube is one object rather than seperate divs!