Kaiser Wilhelm II was reticent to allow the wedding of his cousin Friederic Wilhelm to Pss Agatha of Hohenlohe-Schillingsfürst, on the grounds that she was catholic.
Finally he allowed. Under which religion their children (4 girls) were brought up ?
A dispensation is always required, for a Catholic to enter such a union -- if he expects to remain in good standing with the Church. Otherwise, any marriage contracted without her blessing (yes, since the Second Vatican Council, it has been possible to receive a dispensation of form to marry in accordance with the rites of another church) is considered an *irregular* union, and living in it as man and wife is considered a state of *mortal sin* (but not excommunication, which I used to erroneously think).
We all know what happens to divorced Catholics who civilly remarry (because their previous marriages could not be annulled) or Catholics who marry divorced persons (whose previous marriages lack sufficient grounds for annulments) ...
It's just that the terms required for a dispensation have not been uniform and consistent, throughout the years. When in 1948 the deposed King Michael of Romania found himself at loggerheads with the Vatican, for refusing to promise to raise all prospective children in the Catholic faith, a woman wrote in a letter to the New York Times that this requirement officially became coded into canon law only in 1910, it was strongly resisted in Germany, and the pope at the time had to make an exception for the people there.
But as with so many customs and practices in the Church, this requirement (the expectation of raising all prospective children as Catholics) has always been at least unofficially enforced. I was warned, years ago, that whenever an interfaith union is contracted by a Catholic, the Catholic party typically dominates.
Nevertheless, given the political dimension to European royal marriages, it seems that there has been some leeway with respect to the traditional expectation. A lot of it has to do with place and time: the maternal grandparents of Princess Anne of Bourbon-Parma had been able to obtain a dispensation on the promise that only daughters born to their union would be raised as Catholics.
However, by the time of King Michael's own marriage, the Vatican had tightened the clamp. Thus it was that the bride's mother (Princess Margarethe of Denmark) found herself beating and banging on the desk of Pope Pius XII, demanding that he grant the couple a dispensation -- despite the groom's steadfast refusal to make the promise. One presumes that her Danish cousin, Princess Helen of Greece and Denmark (Michael's mother) was more circumspect, when visiting with her ...
As it was, the couple married in Athens in a Greek Orthodox ceremony, the wedding being a well-attended affair by European royalty -- with the conspicuous absence of the bride's immediate family (she was given away by her Protestant uncle, Prince Erik of Denmark). The marriage was canonically valid and legal in every jurisdiction but the Catholic Church; but even the latter eventually granted her blessing. Evidently the couple managed to finally obtain a dispensation in 1966; so they underwent a second wedding ceremony in Monaco.
Despite King Boris of the Bulgarians reneging on his promise to raise all his own children as Catholics (his wife was born Princess Giovanna of Italy, daughter of a mother who had converted from Orthodoxy to Catholicism), by allowing his first child and only daughter (Princess Marie Louise) to be baptized in accordance with the rites of the Eastern Orthodox Church, Queen Ioanna managed to remain in good standing with the Catholic Church.
Like her husband's mother, Princess Marie Louise of Bourbon-Parma, who was spared from excommunication for Boris being raised Orthodox (after getting baptized Catholic). Pope Leo XIII rightly let her off the hook, and issued the bull only on King Ferdinand (eventually lifted).
King Ferdinand of Romania was also eventually forgiven for allowing all six of his children to be raised Orthodox, despite promising only his heir. It only stands to reason that when practices have not been uniform and consistent among commoners (look at the German ancestors of Grace Kelly), thing have been even more fluid among royals.