Is the son of King George, born to an American mother, loyal to America? Perhaps, but the founders decided it was too great a risk for the highest office of the land. Citizen yes, natural born citizen no.
Let's leave the issue of royalty, with the possible complications of claims on the British throne, out of it for now.
Suppose a British general who becomes dissatisfied with the regime moves to America, marries an American woman and sires a son.
That son is raised as American from birth and never feels loyalty to any other nation.
I think the founders would relate and I see no reason they would want to prohibit that boy from becoming President one day.