None. Subdatasheets are part of tables and queries which don't have events.
Events are associtated with forms, reports and controls. Subdatasheets are essentially a developer tool rather than an element of a well designed production database.
I have never succeeded in displaying a subdatasheet in a form in datasheet mode.
The only way I know to show a subdatasheet is when the Source Object of a subform control is a table or query.
OnLoad, OnOpen, OnCurrent - they are all getting a run. The problem is that they run once when you first open subdatasheet. I cannot figure out what event happens when you expand subdatasheet second time.