I have recently been reading about the 1688 English Revolution, which put William and Mary on the throne, more specifically about William and Mary's roles in deposing her father.
At least one book states that William, Mary, and her sister Anne were declared "Children of State". This is not a term I have met before. I can understand it somewhat in William's case: his father was dead, his mother a foreign princess who could not possibly be trusted with his upbringing. But - Mary and Anne were Charles II's nieces, both presumptive heiresses in line. As such, their futures - marriages particularly - would naturally have been at the will of the king. So what difference did being "Children of State" make?
Note: Wikipedia has nothing, merely redirecting to Ward of Court, quite a different status.
Edited to quote sources
"...the Dowager Princess Amalia von Solms-Braunfels, widow of Prince Frederick Henry of Orange... tried to obtain custody of her grandson, partly as a family duty towards the state, and partly as she sensed her daughter-in-law had little maternal love for the boy. The anti-Orangists, led by Johan de Witt, considered him as a Child of the State, and maintained he should be brought up as a Calvinist and a servant of the Republic." William and Mary" by John van der Kiste 2011
"Each (William and Mary ) was what was then known as ‘a child of state’, whose family life and domestic contentment depended exclusively on plans made by others as to whom they should marry." (and passim) William III and Mary II Jonathan Keates 2015