Notation Conventions for Tree Data Structures
You can use functions. 

Let $node$ be an arbitrary node of the tree.

Then $data(node)$ is data associated with $node$, $children(node)$ is set of child nodes of $node$ and $parent(node)$ is parent of $node$. 

For data associated with $i^{th}$ child of $node$ you can use $data(children(node)_i)$ etc.