worldbuilding bullshit amnesty
The eldest son of the Charmington royal family is always adopted. This is ostensibly to keep a strong, diverse genetic line for the royal family, but in reality it originated when Prince Herbert the First (second son of King Constantine) unexpectedly inherited the throne when his elder brother died, and he refused to divorce his spouse, Sir Henry. Instead, they adopted a son, and created the tradition primarily as a defense for their marriage. However, as King Constantine was married to own his first cousin, and their children had inherited a number of issues, the adoption actually solved more issues than it created, and the tradition continues on to this day.
Charmingdale (home of Prince Phillip and his bride Snow White) is a very old kingdom, with very old laws, and very young rulers. Their rulers are young by design, as the kingdom expects their elders to retire once they reach late middle age. Their oldest king was fifty-two years old upon his retirement, and was succeeded by his twenty year old son. It's not quite the same as a term limit on ruling, but it functions much the same, and the former rulers of Charmingdale have a much more pleasant retirement than most other kings and queens.
The northern kingdoms (including France, home of King Adam, formerly Prince Adam of Rouen-Metz) are typically ruled by smaller Dukes and Princes, who each govern small holdings, rather than larger kingdoms. It creates some issues when neighboring holdings are in conflict, but generally it means that each ruler stays in touch with the needs of their constituents, because their holdings are so limited in size. When Princes get uppity and start conquering each other, the problem usually fizzles out before it reaches a kingdom size, because the farmers and townsfolk don't especially like being governed by random strangers from the next holding over, and they'll rebel if their rulers don't have enough time to listen to them.
Riding (West, East, and Upper) is mostly forest lands. It's barely governed. The Riding government is a group of elected representatives from the villages on the borders of the forest. They're regular people. They serve for typically 4-5 years before passing the position on to the next representative.
Camelot, due to an unfortunate technicality with fairy tale logic, doesn't have a succession plan for their royals. They literally have a Once And Future King. That's him. He's their only one. He's going to rule forever. He'll never die. He literally just Won't Do That. They don't need to plan for his son inheriting the throne, because he's their forever king. They're not worried about it.
The kingdoms around Camelot all have succession plans that involve marrying into the royal family and positioning their children to step onto the throne when King Arthur dies. They're not subtle about it. The Auradon ruling council has plans to promote Sir Gawain's son to king when it happens, to avoid a power vacuum in one of the most magically inclined sovereignties in the USA and also to avoid the neighboring kingdoms throwing an unknown stranger onto the throne.
Atlantica does have a functionally-immortal ruler, but they also have a functioning succession plan. Mermaids follow a rule of inheritance through combat, so whoever can defeat the current ruler of the kingdom in formal combat is accepted as the new ruler. Poisoning is allowed, but is considered unsportsmanlike after King Anemone of the Aegean sea won his throne by using a poison-tipped dart to defeat the former Queen Althea.
