In countries like Sweden or the Netherlands, and I suppose also in Belgium, the regent is chosen by the country's parliament and doesn't necessarily have to be the next adult in line to the throne. [...]
When King Willem-Alexander ascended to the throne (2013) there were even three separate Acts issued (and approved) around the regency:
Regency Act 2013
Regulates that HRH Princess Máxima of the Netherlands will be the Regentess until her daughter, the Queen, has reached the age of 18 years.
Regulates that in an event that also Princess Máxima has passed away, HRH Prince Constantijn of the Netherlands will be the Regent until his niece, the Queen, has reached the age of 18 years.
Act on the annual dotation to the Regent 2013
Regulates that the Regent(ess) will receive a part of the annual dotation to the Queen as an income.
Regulates that this income will be calculated (levelled) with the normal dotation which a Regent might receive as dowager to the late King.
Regulates that certain residences may be given at the disposal of the Regent(ess).
Act on the parental authority over the minor King 2013
Regulates that HRH Princess Máxima of the Netherlands has the parental authority over her daughter, the Queen.
Regulates that HRH Princess Máxima of the Netherlands also has the parental authority over other children born from the marriage of the late King.
Regulates that HRH Princess Máxima of the Netherlands will be advised by a Board of Trustees.
Regulates that the members said Board will be: the Vice-President of the Council of State, the President of the Auditory Chamber and the President of Supreme Court of Justice and two more members to be appointed by Royal Decree.
Regulates that a Secretary will be appointed and a secretariate will be created for the Board of Trustees.
Regulates the costs for the workings of this secretariate and the re-imbursement of the members of the Board of Trustees.
Regulates that the parental authority is limited concerning the properties which are owned by the (underaged) Queen.