This scan of the census shows most, but not all of the 1880 census entries for this family. One can see the birthplace for the children and, assuming the birthplaces of the children are correct, approximate the year the family migrated from Canada to Illinois. In this case, the mispelling of the name would not be enough so that the family could not be located on the Soundex for the 1880 Illinois Census (as NEILL and NEAL both have N400 as their Soundex equivalents).
You can Search the 1880 Census for free
(c) 2001 Michael John Neill
These census images were used with permission from the census image database at Ancestry.com