A man named Nicholas Salisbury was a businessman in Liverpool, England, in the mid-19th century, according to census and Ancestry.com records. He and his wife, Anne, had only one child, a daughter named Maria, born in 1805. Maria married in 1831 and had three children.
By 1841, records show that three children – without Maria – are living with Nicholas and Anne.
By 1861, Maria is back and documented as a widow with three adult children. So it's possible Maria left the children to be raised by her parents and that the child pictured with Nicholas in the painting is not his daughter but a granddaughter.
The mystery remains as to why she was painted out of the portrait.