You can't: events can only be invoked from the class that created them. And Child forms should not know about the MDI parent unless they really, really have to - which they rarely do. Accessing Parents is a bad idea from an OOPs POV.
Instead, create an event in the Child form to which the Parent subscribes. In teh handler, teh Parent can take whatever action it deems appropriate.
See here:
Transferring information between two forms, Part 2: Child to Parent[
^]