Molly O'Brien, a Human child
In biology, an offspring is the product of reproduction from one or more parents.
Humanoids often refer to their offspring as a child (plural: children). While the term generally applies to adolescents, indeed, James T. Kirk once described children as "Little ones like yourselves. They grow." (TOS: "The Apple"); the term may also be applied to adult members of a species, alternatively known as a son (male offspring) or a daughter (female offspring).
Richard Daystrom compared the M-5 multitronic unit he had designed to a child, developmentally. Dr. Leonard McCoy didn't care for the comparison but used it all the same when analyzing the scientist's mindset. (TOS: "The Ultimate Computer")
Captain Picard claimed that children were too young to understand the dangers of reality, and so adults were needed to protect them from harm and corrupting influences. (TNG: "Imaginary Friend")
Data created an android child patterned after himself, a daughter named Lal. (TNG: "The Offspring")
Occasionally, beings have posed as a child to gain insight into Humanity's existence, strengths, and weaknesses. (TNG: "The Child", "Imaginary Friend") The children of Drayan II were individuals nearing the end of their lives, rather than beginning them. (VOY: "Innocence")
Some species did not rely on offspring as a means of continuing the species. For example, the J'naii and the Changelings were two species that reproduced in other ways. (TNG: "The Outcast"; DS9: "The Search, Part II") Prior to the intervention of the USS Enterprise on Gamma Trianguli VI, the planet's inhabitants had no need for what they described as "replacements", as they were forbidden by Vaal. (TOS: "The Apple")
Betazoid Ambassador Lwaxana Troi was the daughter of the Fifth House of Betazed. (DS9: "The Forsaken")
Commander Sisko compared the "behavior" of the station's computer with a child after the computer started to function differently. (DS9: "The Forsaken")
While under the influence of the Saltah'na energy sphere, Jadzia Dax compared her relationship with Benjamin Sisko as being a son or nephew to her. (DS9: "Dramatis Personae")
In 2373, multiple children were seen in a mind meld which Tuvok and Kathryn Janeway participated in. One of the children, a young girl who was desperately clinging onto each of the other children's hands on the edge of a precipice, was actually a manifestation of a memory virus. The children who were holding onto her hand were Tuvok, Janeway, Dmitri Valtane, a 20th century boy who was wearing sportswear, a Masai African boy from Colonial times, a Chinese peasant girl from Medieval times, a Middle Eastern girl from the 1st century, and lastly, a Stone Age proto-Human boy from ancient history. (VOY: "Flashback")