nil IS a list...

ddGetObjChildren( 
b_objId 
) 
=> l_children | nil

When you say l_children, isn't it already implied that the list could be empty?  After all, if the object has no children, then you get nil, which is just a zero-length list of children.  So shouldn't the return type be simply l_children, like this:

ddGetObjChildren( 
b_objId 
) 
=> l_children

Just sayin'....