In the biographical index starting on page 483 of Le Faye's "Jane Austen's Letters"(new edition), it states:

(p. 506) Children family, of Ferox Hall, Tonbridge, Kent. John George Children (1777-1852) married July, 1798 at Pinner Miss Holwell(q.v)

In other words Mr. Children had only one son with the hyphenated name of John-George, hence the joke,"Mr. Children's two sons are both going to be married, John & George--. They are to have one wife between them..."

Also page 537 of the biographical index"

"Holwell, Miss ?Anne. ...granddaughter of Governor Holwell of Bengal. Gov. Holwell was one of the survivors of the Black Hole of Calcutta, and in 1758 published his "Genuine Narrative" of that event."

Hope this helps make sense of this puzzle :).