The children that are mentioned are probably the only ones still living when the will was probated. There is one child, Mariah who can be found with her sister in 1850 on the census. But she is not mentioned in the will. It could be that Lucy married Ninian about 1815 or so as there is a 10 year spread in the ages between Alfred in 1805 and Preston in 1816. But I am guessing.,