Princess Mary's Scottish-born father is looking after the youngsters while his daughter and her husband Frederik accompany the Danish crown prince's father Henrik on an emotional return to his former home in Vietnam.
Maths professor John, who moved to Denmark from Tasmania after his daughter married in 2004, took the kids to see the Hubertus hunt.
A imaginary fox chase, the event takes place every autumn in Klampenborg, north of Copenhagen.
As the siblings stood side by side it was evident just how much Isabella, two, has shot up over the last six months.
The cute tot is rapidly catching up in height to her four-year-old brother, who she is growing to resemble as time goes by.
While the blonde twosome took in the hunt with their grandfather, their parents were guests of honour at a state banquet in Hanoi, Vietnam.
Beside her dashing prince 37-year-old Mary cut an elegant figure in a beautiful tiered gown in this season's hottest shade, nude.