I finally decided on this scrappy placement for the backing. Getting a design with the leftover squares required a lot of thought, but it finally came together...and I'm pleased with the result. I think a 5 year old will like it... 
There were lots of small blocks left over after pieceing the Halloween quilt top, so I'm thinking of doing a mish-mash of the left overs for the backing. I've placed envelope corners (lower left and right) to mark the area I need to fill. Several strips (not shown) have been stitched together, but their placement will have to wait until tomorrow....I'm seeing spots, bats and pumpkins right now.
I think this is the placement I'll keep working on...maybe... 
After it's cured, rinsed and pressed I'll see what designs may develop.