Princess Charlotte and Prince George have a special name for the Duchess of Cornwall and it's not grandma
Apparently, Charlotte and George do not address the Queen and the possible future queen with the traditional 'grandma' and 'granny'.
In the British royal household, Prince George and Princess Charlotte do not need to adhere to royal titles while addressing the Queen, even though she is pretty much addressed formally by mostly everyone in the world. Princess Charlotte and Prince George mostly find themselves being exempt from rules usually followed by almost all other people in the world, even when it comes to the Duchess of Cornwall. Apparently, as per Good Housekeeping, Charlotte and George do not address the Queen and the possible future queen with the traditional "grandma" and "granny."