Child

Children, at least in the case of humans, were those individuals who hadn't yet reached puberty. Sontarans refered to children as half forms. (TV: The Last Sontaran, Enemy of the Bane) Along with humans, at least some Gallifreyans had a childhood. (TV: The Sound of Drums, The End of Time, The Day of the Doctor) The Tenth Doctor counted 2.47 billion children living on Gallifrey at the end of the Last Great Time War. (TV: The Day of the Doctor)

The 456
The 456 used human children as "the hit," presumably a type of recreational drug created by their bodily chemicals, which they said "felt good." They claimed that, once processed, these children lived forever. (TV: Children of Earth: Day Four)

In 1965, the 456 first arrived on Earth. They demanded twelve children, in exchange for a cure for a deadly new flu strain that would soon arise. Jack Harkness delivered the twelve children. The 456 left after releasing the cure, leaving behind Clement McDonald, possibly because he was on the verge of puberty. (TV: Children of Earth: Day Three)

In 2009, the 456 somehow got every child on Earth to, at once, stop, and chant "We are coming." (TV: Children of Earth: Day One) The next day, they chanted "We are coming - tomorrow." The next day, as predicted, the 456 returned to Earth. The children all pointed towards central London, where the 456 had arrived. (TV: Children of Earth: Day Two, Children of Earth: Day Three)

The 456 demanded that Earth hand over one tenth of the children of the world. Frobisher offered one child per million people on Earth. The 456 refused the offer; they demonstrated that by getting the children to chant a number: ten percent of all the children of the world. Jack, with Ianto Jones, interfered and demanded that no children be taken. As he demanded war, the 456 demonstrated their strength by poisoning everyone in the building. (TV: Children of Earth: Day Four)

The next day, as per decisions made by the Cabinet, children were to be taken by bus to be "inoculated." In reality, they were being taken to be given to the 456. In the end, Jack defeated the 456 using the children to broadcast a signal fatal to the 456, but at a great cost: he had to kill his own grandson. (TV: Children of Earth: Day Five)