This is a simple piece that snaps in place and covers the power button. It can stop the light from shining through as well as hinder your child from mashing the power button. It holds in place nicely while still allowing the sensor below the button to be in plain view.
Try it out and let me know if you like it. I'm not much of a designer, just an occasional problem solver.
I printed mine in ABS so it would have a bit of give to it but I'm sure PLA or anything else would work just as well.