The king and queen of May I Bring a Friend? by Beatrice de Regniers. (Every picture of them is absolutely adorable.)
Babar and Celeste of Meet Babar and His Family by Laurent de Brunhoff.
King Albert and Queen Petronella in one of my childhood favorites Septimus Bean and His Amazing Machine by Janet Quin-Harkin.
Any royal couples you can think of to add to the list?