Child seat: how to keep the child's head from falling forward? When my child falls asleep in the child seat, his head falls forward.
How can I keep his head back?

The child seat is a modern type with a back that can be tilted back to prevent that. It works, but only as long as the bike isn't moving. But at speed, the seat's movements from an uneven road surface and from simply pedalling will slowly but surely make his head fall forward. Short of replacing the child seat with one whose back can be angled even further back, what options do I have?

I thought of the whiplash protectors used in Formula 1 cars: somehow strap the child's helmet to the seat back. But that would be neither safe nor effective, as such a strap would only tug at the helmet (not the head) with the risk of slow suffocation or at least a helmet that slides into the neck and would no longer protect the head in case of an accident.

Let me add a fun photo: here, a cloth bag is hung from the helmet in an attempt to keep the child's head from falling forward. We did NOT actually do that, but it shows the idea.
Here is a solution that actually worked very well -- and during our testing it appears safe to use, seemingly also in the event of an accident. (If you disagree, please explain why!)

What we did was to loop a thick soft rubber band around the helmet's headband, and then secure the other end of the rubber band to the seatback using a bit of string through one of the shoulder-strap openings, to achieve a horizontal pull.

Earlier, we tried looping the rubber band through one of the helmet's air vents, but this caused problems: pulling the helmet back towards the neck revealing the forehead, and potentially putting upwards pressure on the chin strap. Evidently, the helmet's shell should be left alone.  

Attaching the rubber band to the helmet's headband was much better. As you can see from the photo, it provides a very level pull (not angled) in the perfectly opposite direction of the force of the falling head. The length of the rubber band also allows sufficient freedom of movement so it doesn't restrict the child when he's awake. Since the headband is already designed to spread the pressure evenly around the head, it also appeared to comfortably spread the "hold-back force" (feel free to edit my lacking English).