Any Mashup which has a Parameter, also has a ParameterChanged event generated automatically.
The real interesting thing is what if you want to raise an event from the ContainedMashup to the Host Mashup.
In this case you can create inside the child Mashup an Expression with a DateTime output, with "new Date()" in the expression field and bind the result to a Session variable which is type DateTime. Anytime you want to trigger the event, just trigger the Evaluate service of the Expression
Then, on whatever mashup you want, you can use the Session variable changed event for event monitoring.
Hope it helps,