WILD  AUSTRALIAN  CHILDREN
HOOMIO AND  IOLA
TOM  AND  HETTIE
aka
This is the only photo I've seen that would indicate the two were together awhile.  But is this the same couple?  I'll let you decide.
Jan 1866 ad
Dated 1862 - 1871 based on the revenue stamp affixed to the back
According to "P. T. Barnum: America's Greatest Showman" by the Kunhardts (1995) the explorers who first
discovered the pair thought they were kangaroos. Their publicity pamphlet referred to them as origially being "sharp-toothed cannibals" belonging to a "distinct race hitherto unknown to civilization."  The authors go on to say that it was all a bunch of ballyhoo, of course, and that the two were simply severely retarded siblings from Circleville, Ohio.  Another source claims they were from Warwick Co, Indiana.  Their original publicity booklet refers to them as Hoomio and Iola but the same booklet reprinted in 1872 calls the Tom and Hettie.  It is not clear if the same couple was referred to by both names.
"Another good pair of fakes were Tom and Hattie.  They were called "The Wild Children."  But they were not wild.  The only time I ever knew them to be wild was when they couldn't go out on the street and play with the other children.  They were born up in the peninsula district of Michigan and were just ordinary children with dwarfed bodies and twisted brains."  - 1911