All good ones, much better than that dreadful Susan one, which is so easily answered by any fan.
DrDeth, by “that dreadful Susan one” do you mean why is Susan no longer a friend of Narnia?
Yes, but of course she is. Just not for the nonce.
"Once a King or Queen of Narnia, always a King or Queen of Narnia.’
It is shown many old people cam back to the fold.
Susan is just going thru a phase.
Historically, Lady MacBeth had a son with her first husband, Gille Coemgáin. His name was Lulach, and he succeeded MacBeth as King of the Scots. He was a weak monarch, and was usurped and killed only a few months later by Malcolm, Duncan’s son. Shakespeare apparently didn’t find his reign (or even his existence, beyond the mention of his being suckled) worth mentioning. 
What did the narrator see at the bottom of the well in Poe’s The Pit and the Pendulum?
I think Shakespeare implies pretty clearly that the fictional Lady M’s child is dead. It’s not explicit, but the child is never mentioned except for that one time - there’s no ‘We’re doing this for the kiddies, darling’ or ‘Little M and his descendants will reign for centuries’ or ‘Well, at least if I get killed Little M will be around to take over’ or ‘Shit, we need to get Little M to safety/at least Little M is safe with your cousins’ or anything else that you’d expect, in a play that’s so bound up with questions of succession and blood, if either of them had a child who could be expected to succeed them. The only thing along those lines is his ‘Bring forth men children only; for thy undaunted mettle should compose nothing but males’ - it’s about their future children, not about existing ones. There are even two other children in the play (Banquo’s son Fleance, and Macduff’s son), which isn’t common in Shakespeare and isn’t really necessary to the plot - their presence is an extra highlight of Lady M’s child’s absence.
From an actor’s or director’s point of view, I think it can be a really strong choice either to play it like the child was both of theirs, or like the child was hers from a previous marriage. That one mention of the child (‘I have given suck’ etc) is clearly her ultimate weapon, the nuclear option she brings out in extremis to force him into the murder. So depending on how she plays it, either she’s saying (among all the other layers of meaning) ‘Another man gave me a child. You haven’t been man enough to do that. Man the fuck up and kill this guy - this murder will be our equivalent of a shared child, the thing we create together’ or else she’s saying ‘You know the most vital and emotionally powerful thing you and I have ever done together? The worst pain we’ve ever been through together? Yeah, fuck that. This murder is more important to me.’
And in Lear, I like to think the Fool vanishes because Robert Armin was double-jobbing and had to do a quick change after Act III and leg it across to the Swan Theatre for his other gig.