It's not equality exactly when hypothetically two grandchildren (through sovereign's daughter/female line) is styled as children of an Earl without HRH (had Captain Mark Philips accepted the Earldom), whilst the other grandchildren were styled as HRH Prince/Princesses (through the sovereign's male line). HRH Prince/Princesses outranks children of an Earl.
Lady Louise Mountbatten-Windsor and Viscount Severn are legally HRH Prince/Princesses under 1917's Letter Patent as children of the son of the sovereign. However, their parents decided that they are going to be styled as children of an earl.
If Mark Philips have accepted the earldom, Peter Phillips will be entitled to use his father's subsidiary title (as the eldest son) and Zara Phillips would be Lady Zara Philips at birth (similar to how Princess Margaret's children were styled, who are also grandchildren of a sovereign through a female and non-heir line). Of course when Zara gets married in this circumstance, she would be Lady Zara Tindall just like how Lady Sarah Chatto is styled. Again, they are not HRH Prince/Princesses, because they are children of the daughter of the sovereign (not the children of the
son of the sovereign). Had Princess Anne and Princess Margaret had more sons, they would be styled as The Hon. [First Name] [Surname], which is below Lord/Lady but above those without any titles. In contrast, if they had more daughters, they would still be styled as Lady [First Name] [Surname] like their eldest sister. Yes, David Armstrong-Jones (son of Princess Margaret) has succeeded his father's earldom and becomes The Right Honourable The Earl of Snowdon in 2017.
To go further back into the generation of George V's children, where the King has five sons and a daughter, who are affected by 1917's LP. Princess Mary, the King's only daughter married Viscount Lascelles (son and heir to Henry Lascelles, 5th Earl of Harewood). Her two sons were The Hon. George Lascelles and The Hon. Gerald Lascelles at their birth (styled as the sons of Viscount Lascelles). Yes, Lascelles is the Earl's family surname. They were not HRH Prince either, despite being the eldest grandsons of George V. When Princess Mary's husband succeeded the Earldom and becomes the 6th Earl of Harewood in 1929, The Hon. George Lascelles became Viscount Lascelles (using Subsidiary title) and The Hon. Gerald Lascelles remained the same for the rest of his life. Of course when Princess Mary's husband died in 1947, her son became the 7th Earl of Harewood.
Pictures of George and Gerald both styled as The Hon. [First Name] Lascelles, in the labels of photographs before 1929
https://www.lookandlearn.com/histor...Mary-Princess-Royal-Viscountess-Lascelles.jpg
https://media.gettyimages.com/photo...en-gerald-and-picture-id502832061?s=2048x2048
Even if Peter Phillips accepts the Earldom, Savannah and Isla would still be Lady Savannah Phillips and Lady Isla Phillips, not HRH Princesses like Princess Charlotte of Cambridge. Similar to Princess Alexandra, The Honourable Lady Ogilvy's situation (as a granddaughter of the sovereign through a male line), where The Hon. Angus Ogilvy (second son of 12th Earl of Airlie) turned down an earldom, had Jack received and accepted an Earldom, August Brooksbank would entitled to use his father's subsidiary title. Princess Alexandra's children also do not have titles: James Ogilvy and Marina Ogilvy.
The latest exception of Prince/Princess given to grandchildren of the sovereign through female line was Princess Alexandra and Princess Maud in 1905 (daughters of Princess Louise and 1st Duke of Fife), but they were Her Highness not Her Royal Highness. Between when they were born and 1905, they were Lady Alexandra Duff and Lady Maud Duff, styled as daughter of The (1st) Duke of Fife. The 1917's LP by George V did not take the HH Princess titles away from them.