...even if they have not paid for a seat. I didn't know this, but according to the website they do:
Infants not entitled to a seat are entitled to one piece of checked baggage weighing no more than 10 kg and with dimensions of 9 x 14 x 22 ins. They are also entitled to one piece of hand luggage weighing no more than 6kg. This is over and above the buggy or bassinet or car seat, which are carried free of charge.