If you have a findmypast sub, you can see H, Mary and Edith M Jackson are in Howden, E Yorkshire in 1921. Four people named Bulmer at the same address.
This rather suggests that not all Herbert and Mary Jane's children will have been born in Easingwold district.
I can see nine possibles with Mmn Bulmer, as well as those you have named (Ethel Mary 1918, Thomas William 1927, Norman 1934):
Alice Margaret 1921 Howden district (I think she is the same child as Alice Margret Jackson who died in Goole district aged 0 in 1922).
Doris 1922 Goole district
Kathleen 1924 Goole district
Ethel 1925 Howden district
Hannah 1928 York district
Charles Henry 1930 Bedale district
Herbert 1931 Thirsk district
Celia 1935 Great Ouseburn district
Elizabeth 1937 Great Ouseburn district
(There are a few Jackson/Bulmer births in Sunderland district during this time period, but I think they belong to a couple who married there in 1930).
The family are (mostly) in Whixley, Yorks. in 1939. Herbert, Mary J, and Elizabeth (birthdate tallies with a birth registered in 1st quarter 1937), plus seven redacted individuals. Oddly, Ancestry's transcript also shows Charles Jackson born 1930, although the image doesn't.
I don't know whether you have seen the 1918 marriage record for Herbert and Mary Jane, but Herbert's father is named Thomas William.