The best device I've used for my son is this:
Safe Airplane Travel for Kids - CARES Child Aviation Restraint System | Kids Fly Safe - CARES | Kids Fly Safe
It is very cheap, works well and from their website you are able to download the approvals for most Aviation Authorities.
I have been challenged by certain airlines, but providing either FAA (or whatever country's authority) letter has usually satisfied those who question it.
It should be noted that this device is designed for children over 22 pounds or about 10kg, My son was that weight at 6 months.