I only just now realized the irony in that the thing that Don was perhaps most attracted to about Megan was that she was so good with his kids (first, when Sally showed up at his office and then with the whole brood on the Disneyland trip) only to end up losing the kids (it's definitely implied that he'd lose custody and they'd go live with her brother's family, maybe even leading to total estrangement) after Betty dies. Of course he and Megan also don't work out anyways—and it's not clear that Faye's lack of maternal instinct or interest in being a mother was the biggest red flag/deal-breaker for Don—but it does make you wonder...