Setting Prefab Children to Active before instantiate

Not a newbie.. just a real slow learner here...

I have a character prefab called "PLAYER" .. it has 4 or 5 "avatars" on it that are set INactive in the editor (unchecked at game launch)... go with me ...

I want to be able to set active only one of the avatars (children of PLAYER) so that the user selects their choice and that avatar is enabled in the prefab and then network instantiated (using PUN).

I've got a reference to the prefab avatar child... but .... setactive appears not to work .... is this by design or am I missing something. Looking for clarification on the syntax or a "no you can't do that because prefabs are "frozen" at runtime".