How to instantiate a child when the other child is instantiated?

Hi,

assume there are two concepts:

concept Concept1 extends BaseConcept
...

concept Concept2 extends BaseConcept
...
children:
aChild : SomeType[0..1]

references:
aRef : Concept1[0..1]

What I want is an 'event listener' that creates an instance of aChild as soon as aRef is instantiated (more precisely: user has selected one from a list of available instances of Concept1 within a solution by using completion menu).

A factory only reacts on instantiation of entire Concept2. Also tried action map but which action to use?

Sounds not so difficult but currently I'm at a loss.

Thx
Holger