A child of a node$v$ in a rooted tree is a node $w$ for which $v$ is its parent. In other words,
a rooted tree possesses a hierarchy from its {[root}} to its leaves, and there exists a unique
path connecting the root to $w$. The last node on this path is $v$, and so we say that $w$ is the child
of $v$ and $v$ is the parent of $w$.