I could've sworn that I've read the decision for this problem before, but my searches won't find it, so I'm going to ask again
My founder rolled a wish to have a child right after she gave birth, and I let her adopt a child while the official gen 2 firstborn was still a baby. Now, the rules only say
The firstborn child of any generation is the targeted immortal. In the event a pregnancy results in twins or triplets, then the first named child is the next immortal.
So I think I'm okay, but did I jeopardize HOF because my targeted immortal is no longer the oldest in her generation?