The Family Search website (free), Ancestry.com (subscription), Variety tradepaper, and an El Paso, Texas newspaper have more info:

• 1900 census: 34 year old Daniel Horne [sic] (born Ireland), 30 year old wife Margaret (born Illinois), 9 year old daughter Cecil (born Illinois), 8 year old daughter Glena (born Illinois), 7 year old son Robert (born Illinois), 5 year old son Willie (born Illinois), and 3 year old son Edmond (born Illinois) are living in Spring Valley, Illinois: https://familysearch.org/pal:/MM9.1.1/MSSW-66J
  Ancestry had the 1900 census takers worksheet: Father Daniel's occupation is "Coal miner".
• 1910 census: 16 year old Robert Horner and 14 year old William Horner are among dozens of teenaged boys (listed as "lodgers") who are living together in Chicago: https://familysearch.org/pal:/MM9.1.1/MKZH-ZXW?cc=1727033
  Ancestry had the census takers worksheet: their location is the "Working boys Home" at 1128-1140 W. Jackson Boulevard, Chicago. The occupation of Robert is "Errand - boy" and William is an "Apprentice - printer" (and those are the same occupations listed for most of the boys at the Home).
  This is the Mercy Home for Boys and Girls in Chicago. The home is still in operation and the following link will take you to their history page - note that their Headquarters is at 1140 W. Jackson Blvd., just west of downtown Chicago: http://www.mercyhome.org/mercy-home-history
• World War I draft registration dated June 5, 1917 for Robert John Horner, 4040 Broadway Av., Chicago. He is single; born September 14, 1895 in Spring Valley, Illinois; occupation is "paper writing, North Shore Review" at 4040 Broadway Av., Chicago; and the registration notes "Physical disability - limbs off" and "Limbs severed": https://familysearch.org/pal:/MM9.3.1/TH-1951-26328-17264-36?cc=1968530
• As mentioned on a prior webpage, there's a couple articles on Horner's 1923 marriage:
  September 27, 1923 Variety noted Horner's Los Angeles marriage to Freda Bohm of Chicago, and "... losing his legs in an accident when six years old."
  October 18, 1923 Variety: "Frieda Bohnn ... was married to Robert J. Horner, a legless movie director, of Los Angeles, Sept. 17."
• Les Adams found the September 26, 1939 tradepaper announcement (shown on the prior webpage) about Horner planning some Spanish language films (but never made).
• 1942 World War II draft registration for Robert John Horner; born September 14, 1894 in Springvalley, Illinois; occupation is "Road Show Exhibitor"; and his home address is 962 N. Wilton Place, Los Angeles. His contact is Wm. T. Horner, 5937 N. Maplewood Ave., Chicago, Ill. (his brother who is mentioned in the funeral notice below). There's a handwritten notation on the registration card: "cancel - deceased 7-29-42 El Paso, Texas": https://familysearch.org/pal:/MM9.3.1/TH-1-16386-40835-52?cc=1861144
• If you sign-up for a free user registration at Family Search, you will find a copy of the Texas death certificate for Robert J. Horner, born September 14, 1896 in Illinois. He is divorced; occupation is "Projectionist"; and his parents were Dan Horner and Margaret Cline (both born in Illinois). He passed away on July 29, 1942 at the El Paso City-County Hospital, and the cause of death was cirrhosis of the liver. Interment was Evergreen cemetery (no city mentioned for the cemetery, but I assume it is located in El Paso).
• There was a death notice and funeral information in the El Paso (Texas) Herald Post newspaper:
  Excerpts from July 30, 1942 death notice: "Funeral arrangements for Robert J. Horner of Hollywood, Cal are pending ... died yesterday in a hospital ... survived by two brothers, William and Ed Horner of Chicago, Ill."
  Excerpt from the August 1, 1942 funeral mention: "Burial was in Evergreen Cemetery."
• It was relatively easy to find Horner's two brothers, William and Ed Horner of Chicago, Illinois:
  1942 World War II draft registration for William Thomas Horner, 5937 Maplewood Ave., Chicago. Born 1896 in Spring Valley, Ill. and his contact is E. D. Horner, 6321 N. Talman Ave., Chicago: https://familysearch.org/pal:/MM9.3.1/TH-266-11820-26004-93?cc=1861144
  1940 census for Edmund D. Horner and his wife and two children are renting at 6321 Talman Ave., Chicago: https://familysearch.org/pal:/MM9.3.1/TH-1942-27865-16991-15?cc=2000219

On Saturday night, May 4, 1935, there was a car accident near San Diego which claimed the lives of four of the five people. Child star Jackie Coogan was injured but lived. Coogan's father, John H. Coogan, was the driver and died at the scene. Also killed were 19 year old Trent 'Junior' Durkin, Charles (or Clarence) Jones (the manager of the Coogan Ranch), and Robert J. Horner. Newspaper clippings mention that this Robert J. Horner was an "actor ... 25 years of age ... a young screen and radio writer". Since Jackie Coogan's father was the driver, two lawsuits were filed seeking financial compensation from Jackie - one was by Junior Durkin's family and a second by Horner's mother.

The Family Search website (free), Ancestry.com (subscription), California Death Records database and various newspapers have info on the 25 year old Robert J. Horner. A consistency across these records and articles is his mother, Anna V. Horner:

• 1910 census: 28 year old Robert J. Hoerner [sic] (born New York), 26 year old wife Anna V. (born New York), 3 year old daughter Elsie V. (born New York), 2 year old daughter Muriel E. (born New York), and newborn son Robt J. (born New York) are living in Queens, New York: https://familysearch.org/pal:/MM9.1.1/M53Y-QYP
  Ancestry had the 1910 census takers worksheet: the occupation of father Robert J. Hoerner is "Merchant - Furniture". The census visit occurred on May 21, 1910 and newborn son Robert J. is 11/12 old (indicating about eleven months of age, which would put his birth year as 1909).
• 1930 census: 46 year old Anna Horner (born New York; widowed), 23 year old daughter Elsie V. (born New York), 22 year old daughter Muriel (born New York), 20 year old son Robert J. (born New York) and 18 year old daughter Constance (born New York) are living in Manhattan, New York, New York: https://familysearch.org/pal:/MM9.1.1/X4KG-XNS
  Ancestry had the 1930 census takers worksheet: the only member of Horner family with an occupation is 20 year old son Robert J. ("Stenographer - Chemical"). They are one of many families renting at the Hotel Dauphin, 67th Street and Broadway, New York City.
• Newspaper articles note that the Robert J. Horner who died in that car accident was 25 years old (which would put his birth date as circa 1909). There was a funeral notice for Robert J. Horner in the May 7, 1935 Los Angeles Times newspaper. Excerpt: "Robert J. Horner, beloved son of Anna V. Horner, loving brother of Muriel, Constance and Elsey V. Horner." (The mother and three sisters mentioned in the funeral notice match the 1930 census info above.)
• The California Death Index for years 1909-1939 doesn't provide much detail - there is a record for Robert J. Horner passing in 1935 and he was born about 1910 and is about 25 years old: https://familysearch.org/ark:/61903/1:1:QKS9-35FY. And there is no info in the Social Security Death Index (SSDI) since this Robert J. Horner died prior to the establishment of Social Security numbers.
• Anna V. Horner's lawsuit against child actor Jackie Coogan ended in June, 1936, and there were newspaper reports. Excerpt: "Judge Swain ... instructed the jury to return a verdict for the defendant on the ground that a minor has no control over the action of his father and hence young Coogan was not responsible for Horner's death." (The suits by the Durkin family were also dismissed.)
• His mother, Anna V. Horner, is specifically mentioned in the accident articles as well as later newspaper reports on her lawsuit against Jackie Coogan. The California Death Records database has a record on Anna V. Horner, born October 12, 1886 in New York and passed away on February 26, 1973 in the Los Angeles area.