It looks like William and Amy had more children - maybe tracing them might help. On the 1900 Census - William and sister Rachel list parents born in Ireland and Scotland?There are Margaret (PA) and Joseph McMullen(IRE) in the 1850 census with son Alex of the correct age, but his sisters are Ann and Margaret.
US Census, 1880 Levee, Pike, Illinois
Citing this Record: "United States Census, 1880," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MXKW-48S
Self William Mc Mullen M 45 OH
Wife Ammie Mc Mullen F 41 OH
Dau Nancy C. Mc Mullen F 18 IA
Dau Margrett A. Mc Mullen F 11 IA
Son Jasper L. Mc Mullen M 8 IL
Dau Cora E. Mc Mullen F 4 IL
US Census, 1900 ED 118 Payson township Payson & Plainville villages, Adams
Citing this Record: "United States Census, 1900," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MSSM-279
Head William A Mcmullen M 65 Feb 1835 Ohio /IRE/Scot
Wife Amy Mcmullen F 60 Apr 1840 Ohio
Sister Rachel Mcmullen F 68 Apr 1832 Ohio /IRE/Scot
Dau-in-law Mystle Mcmullen F 22 Jan 1878 IL
Granddau Daras D Doyle F 3 Dec. 1897 IL
US Census, 1850 Liberty, Guernsey, OH
Citing this Record: "United States Census, 1850," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MXS1-GS4
Joseph Mcmullen M 55 Ireland
Margt Mcmullen F 44 Pennsylvania
Ann Mcmullen F 19 Ohio
Alec Mcmullen M 18 Ohio
James Mcmullen M 16 Ohio
John Mcmullen M 14 Ohio
Robt W Mcmullen M 11 Ohio
Margt Mcmullen F 9 Ohio
Joseph Mcmullen M 7 Ohio
