Hi Steve,
Thanks for contacting Syncfusion support.
We don’t have a built-in support for a button within the node to expand/collapse the node and we have provided a support for Expand/collapse the parent node by using Node’s “IsExpanded” property.
However, we suggest you to customize the any Diagram’s shapes to create the shapes with button by using override the “Draw()” method in customized class. Please refer to our Expander sample which is installed in below mentioned location in your machine.
[InstalledDrive]:\Users\Public\Documents\Syncfusion\Windows\[InstalledVersion]\Diagram.Windows\Samples\Getting Started\Expander
Regards,
Naganathan K G