I had a similar problem when I used to go for walks in the woods around our home when my son was little, mounted in a backpack child carrier. He would sometimes be lulled to sleep by the rhythm of the walking, so we'd have to stop, as his head would loll to one side. But if I turned round to see if he was nodding off, my twisting shoulders would push the backpack out of view.

My solution was to stick up a tablespoon from the kitchen in my side pocket as we went out, and hold that up as a convex mirror, offering a perfect view. Not practical when riding a bike, of course, but a useful tip for those solo-hiking with back-mounted kids!