Found some answers for you.Yes there was a son Andrew jr bn 1923 in CA to Andrew & Verda.The Robert you mention seems to be the Robert J. 2 bn CA with parents, Lawrence F. & Edith D. Blankenship living in CA, San Diego for the 1930 census.
Name:BLANKENSHIP, VERDA LENOR Social Security #:562341690 Sex:FEMALE Birth Date:24 May 1891 Birthplace:NEBRASKA Death Date:12 Sep 1982 Death Place:SAN DIEGO(County) Mother's Maiden Name:SCOTT Father's Surname:MOORE
Name:Andrew Blankenship SSN:700-12-1598 Last Residence:92045Lemon Grove, San Diego, California, United States of America Born:13 Dec 1880 Died:Mar 1978 State (Year) SSN issued:Railroad Board (Issued Through) (Before 1951 )
**Interesting here is his date of birth. On the 1900 census it is Aug 1880 BUT on his WW1 Draft Registration card Andrew Jackson Blankenship, wife Verda Leonor, he isliving in California, states he was born 13 Dec 1880.**
Name:JOHNSTON, MILDRED LAURA Social Security #:571868987 Sex:FEMALE Birth Date:4 Feb 1911 Birthplace:TEXAS Death Date:27 Jul 1986 Death Place:SAN DIEGO (County) Mother's Maiden Name:MOORE Father's Surname:BLANKENSHIP
1920 CA, L.A. Co, Redondo Blankenship, Andrew39 bn TX , section foreman RR , Verda L. 27 bn NEB , Lawrence F. 11 bn TX , Mildred L. 8 bn TX , Helen F. 4 8/12 bn TX
1930 CA, San Diego Co, San Diego City, San Ysidro twp Blankenship, Andrew 41 md @19 bn TX , Verda L. 38 Md @16 bn NEB , Helen F. 14 bn TX , Andrew jr. 7 bn CA Andrew is still a section foreman on RR
1930 CA, San Diego Co, National City Blankenship, Lawrence F. 22 bn TX , Edith D. 20 bn ILL , Robert J. 2 bn CA Lawrence is a section foreman on RR