Relationships and titles can be tricky for any family, let alone the royal family. In fact, I can't imagine Prince William or Prince Harry calling HM Queen Elizabeth II "grams" or "GG" — she is the matriarch of the whole country, after all. But hey — what do I know?!
However, for married-in step-relatives like Prince Charles' second wife, Camilla Parker Bowles, I suspect it gets a little trickier for family members, especially young children, to navigate her official unofficial title.