Bear in mind that customising both parents is largely a side effect of being able to select (and customise) either, in order to play as that parent's six. Making the child a mix of the two only makes sense if the player decides to change the race of their character.
True, they could have gone with "first choose the six of your character, now that's fixed and unchangeable so choose the appearance", but that would have been... rather high-handed and unfriendly. So they allow the player to switch backward and forward between player-character sixes while customising appearance. Little (if any) extra work on their part, and a present to those players who think "But my character would never marry someone who looked like that!"
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By the way, I'm not firmly of the opinion that the spouse and child are definitely dead. Just that the customisation thing doesn't really tell us anything.