My understanding of how this works with child programs is that
Cast triggers Program A. If for some reason the lead is EXCLUDED from Program A, then it will receive nothing until Cast 2.
Programs triggered by a Cast run independently of the Engagement, that is, you could get into Program A on Cast 1 and still be receiving Program A's content when Cast 2 occurs.
Now, if your Child Program already had Bob in it and one of the campaigns already had Bob as a member and the campaign was set to Qualified Once, then Bob would NOT receive the content the second time. This is different than the Engagement Program knowing that Bob already received Program B and skipping that Program in the Cast.