How well did King Harald V of Norway know FDR's dog Fala?

FDR was good friends with the many other Royal families of Europe. Prior to WWII with the back drop of the 1939 New York Worlds Fair, FDR invited several Royal couples to the US as a sort of charm offensive against both the US's neutrality laws, as well as long standing US isolationist sentiments. These visits included family gatherings such as picnics at FDR's home at Hyde Park. (yes with hotdogs).

Royals who partook included Norway’s Crown Prince Olav and wife Martha, the Crown Prince and Princess of Denmark, and King George VI and Queen Elizabeth of the United Kingdom.

According to American Public Broadcasting Service or PBS (America's BBC)

• Roosevelt had a Warm relationship with Queen Martha for six years. From her initial visit in 39 with her husband up until FDR's death in 1945
• FDR was involved in bringing Martha and her children to America for refuge
• FDR hosted them for a time at the White House, and helped them find a place of their own nearby.
• FDR went on to visit with Crown Princess on more than 100 occasions over the course of the war.

These "visits" were typically social affairs like Dinner and Drinks at the Whitehouse and involved Elenore and other friends along with White House co-workers. Several of whom lived at the White House.

• Missy LeHand, FDR's secretary
• Harry Hopkins, FDR's closest friend and adviser
• Sara Roosevelt, the president's mother
• Anna Roosevlet, the Roosevelts' daughter,
• Lorena Hickock, Elenor's friend
• Joe Lash, another friend of Elenors

These along with Princess Martha were constants, but the gatherings were often augmented by new folk.

FDR liked to host informal parties in which he played the bartender as well as the chief. ( he would make a show of toasting bread for his guests ).

The Roosevelts had a similar relationship with the Netherlands Princess Juliana and her children, although they settled in Canada they would often visit with the Roosevelts at Hyde Park. FDR was the GodFather to Juliana's fourth child Margriet, who was born in Ottawa in January 1943.

According to PBS both exile families would receive war news from FDR as well as a sounding board for petitions on behalf of their countries.

sources:

Franklin Roosevelt and Crown Princess Martha: A Friendship Forged in War

No Ordinary Time

This Norwegian site (Royal House of Norway) states that:

While King Haakon and Crown Prince Olav stayed in London, the Crown Princess lived with the children in the outskirts of Washington, DC, until 1945, when peace was declared.

This would presumably be definitive.

It seems that Crown Princess Märtha and her three children, including prince Harald, didn't stay at the White House for very long. Several sources comment that they initially stayed at Roosevelt's Hyde Park estate, then (briefly) at the White House, before moving to Bethesda. For example, an article in the Washington Post observes:

They stayed briefly at [Roosevelt's] Hyde Park estate and then at the White House, before they settled into a Tudor-style mansion north of Bethesda, off a small road called the Rockville Pike.

It seems that while they were living at Bethesda, Harald attended the Whitehall School.

They arrived in New York aboard the American Legion on 28 August.

From late 1940 until 1945 the family lived in a house on a 105-acre estate named "Pook's Hill," in Bethesda. Several sites state that they lived there from October 1940, but do not quote sources. They were certainly living there by 12 November 1940, when the Pittston Gazette, and other newspapers, reported that President Roosevelt had visited for tea after laying the cornerstone of a new naval medical building at Bethesda.

(That property had been purchased by the Norwegian government and would be sold in 1946.)

So, they could not have lived at the White House for more than 2 months (late August to late October). It was probably rather less.

The Norwegian Royal family had been invited to the US by President Roosevelt. As such they would have been considered to be his guests. They would probably have been invited to the White House fairly often. He certainly seems to have visited them fairly often.

We have photographic evidence that Harald met, and played with, FDR's dog Fala on at least one occasion. Probably more often in fact. However, as to how well he knew the dog, I'm not sure that was ever recorded by history.