Alberto Ferrari published an interesting blog post regarding how to write particular calculations over a parent-child hierarchy in PowerPivot Denali (and in BISM Tabular, of course). One case is the Sum of Leaves (summing only the leaves of a parent-child hierarchy) and the other is the Sum of Children (computes all the nodes that are descendants of the current node, excluding the value of the current node).

It is always more interesting the fact that DAX can perform calculation over hierarchies, without having specific commands to navigate in a hierarchical structure. The results is not so easy to write, but it works and is probably pretty efficient.