Using "Timeout" as a severity keyword for the ChildrenFilter is currently not possible. It's an interesting possibility for a future feature!

I figured out a workaround, brace yourself:

• Create a correlation rule that automatically triggers on timeout alarms
• As an action, execute a script that takes the element as a (dummy) parameter.
• Add another action that also executes a similar script, but on clear
• In the script actions, execute a c# script that sets a property on the element
• Add a ChildrenFilter on your child shape that triggers on the property value

There might be simpler ways, but this is the first thing I could think of.