You have missed out a couple of Isabella's children that I know of from the 1911 Census (all Presbyterian, born Co.Derry):
Bella Morton, 67, Female, Head of Family, Read and write, Widow, m.49 years, 12 children/9 living
Francis Morton, 48, Male, Son, Farmer, Read and write, Single
Annie Morton, 39, Female, Dau, Read and write, Single
Minnie Morton, 37, Female, Dau, Read and write, Single
Lizzie Morton, 31, Female, Dau, Read and write, Single
I was hoping that David was one of the ones not listed - note at the time of the 1911 census she had 12 children of which 9 were still alive. I do wonder whether this family is related to mine due to the name Francis popping up in it.