\begin{definition}[Definition:Rooted Tree/Child Node]
Let $T$ be a rooted tree with root $r_T$.
Let $t$ be a node of $T$.
The '''child nodes''' of $t$ are the elements of the set:
:$\left\{{s \in T: \pi \left({s}\right) = t}\right\}$
where $\pi \left({s}\right)$ denotes the parent mapping of $s$.
That is, the '''children''' of $t$ are all the nodes of $T$ of which $t$ is the parent.
\end{definition}