I don't think grandparents would ever be considered as godparents...they are grandparents and give guidance and wisdom in other ways and being older also hinders their ability to be there when the godchild comes of age...I think the godparent is usually someone of the same age group as the parents or younger, someone with similar values to the parents and someone who can offer a insight into the character and life of their parents (which is why it is important for Mary to always choose an Australian godparent)...
So I really don't see grandparents, John Donaldson, and wife Susan Moody and QM and Prince Henrik EVER being considered for this role at all...I see Patricia Bailey being chosen as (one of) the girls godparents and John Donaldson jnr being chosen for the boy...then all siblings (of Mary) have a godchild and make up the Australian criteria...or vice versa
There is royal precedence. Infanta Leonor of Spain's godparents are the King and Queen of Spain, her grandparents.
Prince Felipe's godparents are not only his grandfather Infante Juan, Count of Barcelona, but his Great-grandmother Victoria-Eugenia.
Prince Willem-Alexander's godfather includes his grandfather, Prince Bernhard (it seems in the Netherlands there is also a tradition for a godparent to be a member of parliament, both he and his daughter Catharina, have)
The Duke of Brabant, Prince Philippe's godparents include his paternal grandfather King Leopold III, and his maternal grandmother Donna Luisa, Princess Ruffo di Calabria.
Crown prince Hakon has his grandfather Olav V,
Ingrid-Alexandra's godparents include King Harald her grandfather, and her maternal grandmother
Sverre-Magnus has his grandmother Queen Sonja
Princess Eugenie of York has her maternal grandfather's second wife as a godmother so step grandma (Kind of like Susan Moody)
Charles has his grandfather, Queen Mary (great-grandmother), Prince George of Greece (his granduncle), the dowager Marchioness of Milford Haven (his paternal great-grandmother), and David Bowes-Lyon (his maternal great-uncle). Not to mention King Haakon VII who was of age of his grandfather. (there were several younger ones as well)
Andrew only has among his much shorter list of godparents than his brother, his granduncle Duke of Gloucester (so brother to a grandfather).
Anne has her maternal grandmother the queen mum, Princess Andrew of Greece and Denmark (paternal grandmother) Earl mountbattan of burma (granduncle),
Edward is the only one without older godparents from Elizabeth's kids
So while I doubt they will, I stated in another post my thoughts on possible godparents, there is plenty of precedence for a grandparent, or a great/grand aunt or uncle (sibling of a grandparent), to be godparent.
Hey not just royal, my godmothers (for some reason 2, not a godfather) are my grandmother's two eldest sisters. They are both alive today, and I'm 30.