There's an option in the settings to "Include CPU (and other) usage of children in collapsed processes." This behavior should be applied to the CPU, Private Bytes, and I/O History graph columns.
For instance, It's only when I expand the parent process entry can I see the CPU history of a Chrome child process, as shown in this screenshot:
Chrome is using 13.57% CPU but the CPU history graph is virtually blank.
The graph for the parent process should equal all of the children's graphs added together.