The reason that I mention this is that Elly has never once even thought of going to a convention. I'm not sure if she means what she says about being thought of as being nothing more than John's little wifey is too degrading to contemplate of if Anne is right about her not getting a sitter but what I am sure of is that she never seems to have even seriously thought of it.
This seems rather odd when you consider that she thinks that the whole point of conventions is to lure John away from her visual range so that he can cheat on her. Her innate fear of being alone is more than likely marching in lock-step with her fear that he's found someone better and she'll be replaced. This, I should think, is why she's so antsy when he leaves and wants the children to fret as well. Since she's half-convinced that he's not coming back, she wants the kids to think this way too.
That being said, it would seem that in this instance, we're supposed to think that Elly is full of crap. That's because we contrast her wild fantasies of John engaging in horrid acts of debauchery with the dull reality of our lad enduring long, boring meetings, hotel food, insolent roommates and a longing for home and family. This is because Lynn actually went to conventions and found herself with the sort of nothing to do that Elly tells her kids that they'd have faced in their holidays in the Sun.