In autumn 1903 Childers travelled to the United States as part of a reciprocal visit between the Honourable Artillery Company of London and the Ancient and Honorable Artillery Company of Massachusetts of Boston. At the end of the official visit he elected to remain and explore New England on a hired motor cycle. One day by chance the machine broke down outside the Beacon Hill home of Dr Hamilton Osgood, a prominent physician in the city. Childers diffidently knocked to borrow a spanner but, as a visitor with the celebrated HAC, he was invited in for dinner and introduced to Dr Osgood's younger daughter, Mary Alden ("Molly") Osgood. The liberal English author and the well read republican heiress found each other congenial company. The hospitable Dr Osgood organised the rest of Childers's stay, with much time shared with Molly, and the pair were married at Boston's Trinity Church on 5 January 1904.
Childers returned to London with his new wife and resumed his position in the House of Commons. His reputation as an influential author gave the couple access to the political establishment, which Molly relished, but at the same time she set to work to rid Childers of his already faltering imperialism. In her turn Molly developed a strong admiration for Britain, its institutions and, as she then saw it, its willingness to go to war in the interests of smaller nations against the great. Over the next seven years they lived comfortably in their rented flat in Chelsea, supported by Childers's salary—he had received promotion to the position of parliamentary Clerk of Petitions in 1903—his continuing writings and, not least, generous benefactions from Dr Osgood. Molly, despite a severe weakness in the legs following a childhood injury, took enthusiastically to sailing, first in the Seagull and later on many voyages in her father's gift, the Asgard. Throughout their marriage Childers wrote frequently to his wife and his letters show that the couple lived in great contentment during this time. Three sons were born: Erskine in December 1905, Henry, who died before his first birthday, in February 1907, and Robert Alden in December 1910.