Steven Carter

Steven Carter was the son of Joe and Alice Carter and the grandson of Jack Harkness and Lucia Moretti. Being a young boy of school age, he was affected by the 456's invasion. He believed Jack to be his uncle, and was as yet unaware of Jack's immortality and agelessness. The two appeared close, although their contact was infrequent. When Jack needed a child to defeat the 456, Steven was used as the child to reverse the signal. The noise of the children's scream killed him but destroyed the 456, which saved the Earth's children. His mother cradled his dead body as Jack watched.

Behind the Scenes
This was the first time, including the new seasons of Doctor Who, that a child has been killed