There is a huge range, but you must be very careful with them: we know practically nothing about how safe they are. You must consider that the headrests in the car are not designed for comfort but are a key element in passive safety and should be adjusted to the height of every passenger to avoid whiplash and minimize spinal injuries caused by rear end collision and rebound in frontal or lateral collisions.
Child seats are designed to accomplish the same function, so it is important to read the manufacturers instructions to be fully aware of the safety features of each one and only use approved accessories. The majority of accessories are sold by general retailers, unless clearly stated, they are not usually approved to the ECE R44/>04 o ECE R129, European standard, so their safety cannot be guaranteed. Normally only the accessories sold directly by the manufacturer of the seat are approved.
If you wish your child to travel safely and comfortably, we recommend that you choose one with decent lateral supports. Not only do they offer better protection in a lateral accident but also they offer more support for comfort. According to the carmaker Volvo, the fact that a child’s head can slip to the side when they are sleeping is neither uncomfortable nor dangerous. On longer journeys, a good idea is to recline the seat a little so that the head is resting on the back of the seat: assuming the seat allows for this (check the instructions to find out).
Where babies are concerned, it is worth repeating that there is a danger of respiratory obstruction when the head falls forward (the chin on the breastbone). Should this happen, it is necessary to adjust the inclination of the baby seat, following the manufacturers instructions.
Travelling when your children are comfortable is much more bearable, but for their safety avoid putting your children at risk by using untested accessories. If you wish to avoid the head falling too far to one side, the best advice is the adjust the width of the headrest on the baby or booster seat, should it have one and is adjustable.