Oh, i didn´t know that Isabella was already named after her grandmother, neither that Prince Paul was already godfather for Christian. I then propose the name Catherine Mary Elisabeth Ann for the little princess and John Eric Francis Alexander for the little prince; as for godparents i choose the hereditary couple of Holland for her and Felipe and Letizia for the little boy...
Isabelle is Isabella Henrietta Ingrid Margrethe
Christian is Christian Valdemar Henri John
Friedrick will most likely be a middle name for the boy. The royal princes always seem to have either Friedrick or Christian. Joachim has Christian.
Joachim's two older sons both do as well, Felix actually shares three names with Christian. Henrik has Carl but just as a middle name.
So grandparents Henrietta and John (Mary's) and Margrethe and Henri (Fred's) have been honored. Ingrid for Fred's great grandmother. If John is used again, I'd prefer as Johanne for the little girl. Since both Mary's father and brother are named it, could be argued honoring both.
Some nice family names not used yet for the little girl for 2nd,3rd,4th
Renee (Henrik's mother)
Elizabeth (Mary's middle name, her grandmother, as well as HH Princess Elizabeth, Margrethe's cousin)
Anne/a- Anne-Marie of Greece as well as other Danish royals
Mary- For Princess Mary
Patricia or Jane- for Mary's sisters
Catherine, Margarat, Lovisa: great grandmothers of Mary and Fred
Johanne: feminine of John which is found in both families
some nice family names not already used for Christian:
Peter and Archibald: Mary's grandfathers
Andre: Fred's paternal grandfather
Joachim: for Fred's brother
Charles/Karl: name found in both family trees
Knud: Margrethe's Uncle who would have been king if not for constitutional change
Perhaps since the couple's parents were already honored
Princess Anna Elizabeth Mary Renee (Anne-Marie, Elizabeth and Renee for great grandmothers, and Mary for her mum)
Prince Karl Peter Friedrick Andre (Karl family name, Peter and Andre for great grandfathers, and Friedrick for dad)