Mystery Boy

Recently I located my Reed family on the 1855 Illinois State Census. To my bewilderment, an extra boy resided in the household that year. Unfortunately, the census gives the name only of the head of household, and everyone else appears as a hash mark in an age column. The extra boy is under 10 years old. This family did have a son, Thomas B. Reed, who would have been that age in 1855, but family records show that he died before then. So is our death date for Thomas incorrect, or was someone else living with the Reeds that year? I have eliminated the other possibility of a previously unknown child. Why? Because the mother reported in 1900 that she had borne 11 children, and all are accounted for.