I don't think Obi's child would be unknown to him once he's a spirit. If he can find Luke on Hoth or Endor it would be silly for him to not know about his kid. Its just not a good fit for his character or the existing story. I hope they go another direction. 30 years after ROTJ would give us plenty of time for some cool Solo and Skywalker Jedi along with a number of others from other worlds. I don't see any need to tap into Obi-Wan genes.