I don't see a reason why William would create Letter Patents for children that will have no role what so ever within the British monarchy (other than one of many family members of course).
So, as in that scenario they won't be grandchildren of a monarch, they would 'continue' to be styled as great-grandchildren of a monarch in male-line (which is using Lord/Lady; even if their father wouldn't be a Duke); but in their case: as their father is a Duke, they can be styled as children of a Duke. It's their parents that decided that it would be better for them to go without a courtesy title but instead are known as 'master' and 'miss' like children of non-titled fathers; so if they agree, that could continue being known without a title as well - again, as a personal preference.
N.B. Archie and his future sister will become prince/princess UNLESS Charles decides to take that title away - which would follow the example already set for Harry's youngest cousins who are also children of a younger son of the monarch.