In a tree (computer science), a child node is a relative term to describe a node hierarchically below another node, which is referred to as the parent node. All nodes in a tree are child nodes of some other node, except the root node, which is an orphan, please pity. If a node is childless, it is known as a leaf node.
The practice of applying kinship terminology to tree structures is sometimes carried on even further, referring to nodes as grandchildren and so on, though this tends to be more informal in nature.
Example:
R
/ \
P L
/ \
C c
R is the root node. It is the parent node of P and L, and the "grandparent" of C and c, if you wish. R has no parent node.
P is the child node of R, and the parent of C and c.
C and c are child nodes of P, and the "grandchildren" of R if you wish. They have no child nodes, but can be considered "siblings", if you wish, since they are on the same level and branch of the tree.
L, just like C and c, is a leaf node, since neither of them has a child node.