It is considered a child, the person that has up to 12 years of age (inclusive), at the age of checkout.

Half-board and Breakfast supplements are available at reservation checkout.
Half-board supplement already includes breakfast and dinner.
When Half-board or Breakfast supplement for adults is added to the reservation, that supplement is free for children up to 4 years old.
Children up to 12 years old pay half the adult price (50% discount).