Regarding Aliki... (one illegitimate) daughter of Paul I:
There is no "citation" for this information, since it is a personal experience based on partial presence.
A detail: This "daughter" was tall enough for her age to be mostly isolated by her peers. Of course, if one knew of her real father, her sort of unusual height was/is? completely normal.
Now, re: Frederica and Yugoslavia:
One may search the daily press of the period and find answers.
Yugoslavia was certainly not quite communist then. She became "communist" after American interference and influence were rejected by a mostly self-serving nationalist, Tito, in half hearted favour of the U.S.S.R.--which was never allowed to play quite the active role that she did in all other eastern European nations.
For the sake of information, any "sort of civil war"! never held back official visits for all kinds of reasons, especially to a sister country to Greece, like Yugoslavia, unlike to repeatedly pro-German and fascist/communist Bulgaria and Turkey.