Ahhh I looked it up and I see we have the same protocol here in the UK. You're right regarding the situation in America.
But are the children of foreign Royalty also subject to the same law I wonder?
When H&M got married was there any sort of prenuptial agreement regarding custody and travel arrangements of children in the event of divorce, or is there any law regarding this?
It looks like if they ever did split up, which I think to be unlikely, things would be very difficult. As it is, it seems somewhat strange that children with foreign royal titles would still be subject to a parents permission to travel to the country that literally bestows such titles.