Who is this?

[RJNorton]

That could be, Gene, but the Republic magazine was quoted as saying (with regard to Robert's children), "The second is a boy named Abraham, about eight years old, who rides a bicycle like a professional in company with the sons of President Garfield and Attorney General [I. Wayne] MacVeagh." Both the boulder at the left and the backdrop are studio props.

Source: The Lincoln Family Album (Mark E. Neely, Jr. and Harold Holzer), p. 144.