In fact there were a lot of children of the marriage, although a great many diedf young. These are listed in a booklet entitled The Ancestors and Descendents of Alpin Grant privately published in Aberdeen in 1905. Later children are not in the christening record, but there were at least John 1818, May 1819, William abt 1821, Mary 1823, Donald 1825-1825, Jane Elizabeth 1827-1914, Anne abt 1830 and Isabella abt 1831. Some of these children appear in the census records. As John Fraser was an agent of the Bank of Scotland, I am guessing that he moved around from time to time and that the later children were born in some jurisdiction whose records have been lost or are not indexed.
Helen Grant would have been 47 when the second Anne was born, old enough but not out of the question. I have traced descendents of Mary and Jane and we are all confident of the line from John Fraser and Helen Grant. They also had two Johns, two Donalds, and two Helens, so naming a child for a dead sibling was something they clearly did.
The first Ann apparently had a child (or perhaps children) but I would like to know what happened to him/her/them -- died in infancy? grew up and married? As Ann married in 1822 and presumably died just before 1830 (my reasoning -- if she had died before 1827, the parents would have named the next daughter Ann instead of Jane Elizabeth), she could well have had several children. The booklet above merely states "had issue". So far the booklet has been pretty accurate, and it is annotated in the handwriting of my father's aunt so if there were errors that she was aware of, she would have noted them.
Thanks for your help.