The Queen of the UK is also Queen of 15 other Commonwealth Realms and if she were to die her successors would automatically become the sovereign of those nations as well as the UK. What happens if the person in underage? I know British law provides for a Regent whenever the monarch in under 18 but would that regent be able to perform the functions of each commonwealth realm’s monarch too? Would the regent be able to appoint a Governor-General on a Prime Minister’s advice? Would the regent be Head of the Commonwealth or the monarch be? What if the British gov’t appointed a Council of Regency instead of a single Regent?
The Crown in British use is principally vested in “the-Queen-as-advised-by-her-counselors” – i.e., while she is the figure formally taking a given act, her actions are circumscribed by what she is “advised” – royal-courtesy-use for told – to do by her ministers, or in rare cases by her Privy Counselors. While there are rare and unusual circumstances in which she is free to act as Head of State making her own decisions, they crop up substantially less often than, say, formally impeaching the President of the U.S. does.
In her other realms – Canada, Australia, etc. – she is represented by a Governor General, a senior statesman nominated by the Government of the time, who then acts in persona Reginae to discharge her Head-of-State duties for that realm.
A Regent for a minor monarch, whether an individual or a Regency Council, is in fact circumscribed by the same standards.
The monarch, functioning as figurehead, is always head of state of every realm which recognizes him/her as monarch and head of the commonwealth as an institution. If Wills, however, were to marry tomorrow and knock up his bride on their wedding night, then he, his father, and his grandmother were all to die within the next year, the fact that the crown would descend on an infant of a few months would make absolutely no difference – actions would be taken by whoever serves as Regent (individually or corporately) and by Governors-General in the name of that baby, just as though he/she were a mature and seasoned monarch capable of ruling. It’s largely a legal fiction – tempered by the fact that in point of fact, an elderly woman quite aware of her own experience and her own limitations does act “behind the scenes” when she sees it advisable to do so, in the interest of stable, non-newsworthy governance.
But would each realm (Canada, New Zealand, Belize etc) need to pass it’s own law allowing the Reget(s) to perform the monarch’s duties as the relate to that realm or would the British law be enough?