"But there is one new thing that I'd like to experience. Not now. Experience. And that one thing is impossible." (9.252)
Mrs. Frankweiler may be a rich old lady with tons of nice things (that Claudia takes notice of right away), but that doesn't mean that she has everything she wants. She wants to be a mom, or at least to know what it feels like. Since Claudia and Jamie's mom has a few other kids on her hands, maybe she can just take this duo on as her brood.