Are you sure it was WWI and not Spanish American War? If birth was 1866 then he was too old to be required to even file a Draft Registration Card.
There were 3 Army Divisions that each had 10 Corps Units that each of the 10 Corps had Divisions, that then had Companies. So unless you have a Service Number or more information as to state enlist and served from it will be nearly impossible for any information to b found. The NARA had a fire in 1973 that destroyed 80% of Army records from Spanish American War to early 1960's. (Air Corps and Air Force of later years, no Navy or Marine affected). So if daughter asked for records before 1973 then better find what was sent back to her.