The number of subordinates of node is the size of the subtree rooted at , minus (do not count itself). Define ext{size}[v] = number of nodes in subtree of (including itself). Then subordinates[v] = ext{size}[v] - 1.
This converts the problem into a standard subtree aggregation: compute sizes, then subtract one. This is a common pattern. When the problem asks for something slightly different from what you compute, adjust at the end.