The reason why Rachel and Kirsty haven't aged into adulthood (in-universe, since in real life it's because of the main target audience), despite at least 14 years having passed since their adventures began, if the Christmas specials are any indication.
Some powerful magical force, be it the fairies, Jack Frost or something else external, if deliberately keeping the girls and anyone associated with them young (or at their present age) whilst also ensuring no-one in the human world bats an eyelid at it.- If it's the fairies: King Oberon and Queen Titania may fear that, upon growing up, Rachel and Kirsty will stop believing in magic and forget about their adventures. To this end, they may be using their own magic to keep them young. The eventual goal of this could be:
- So they, somewhat morbidly, can hold on to their treasured human heroes forever without worry of losing them to adulthood.
- So they can ensure Rachel and Kirsty have the proper experiences to ensure that, when they eventually do grow up, they won't forget.
- If it's Jack Frost: He's deliberately using his magic to keep them young and more vulnerable so that, when he's ready, it will be easier to remove them from the equation completely. Oberon and Titania may or may not be aware of this, but I'd assume that, if this is true, they aren't and are puzzling over the girls' lack of ageing.
- If it's some external force: The girls have a greater destiny or purpose in the grand scheme of things. We may not ever see it in the books, but it's waiting out there. However, whatever force is keeping them young is doing so either to:
- Prevent the girls from reaching their destiny.
- Ensure they have had the necessary experiences to achieve their destiny when it finally comes around.
- Rachel and Kirsty are in a coma and have been since the first series of books.
- So... one of them? If it's both, that still means there's probably something magical or supernatural afoot.
Before Rachel and Kirsty came along, the human world was a Reassigned to Antarctica situation.
- It's been a long time since this troper read the Rainbow Magic books, but when she did, she believed that before Rachel and Kirsty saved the Rainbow fairies, the human world was a punishment post for the fairies. It (To her,) explained why Jack Frost sent the Rainbow Faries to Rainspell island in the first place, it was originally used to send any fairy that was too good at his/her job, or screwed up really badly so that she/he was out of the way. She also believed that Queen Titania sent Jack Frost there. However, this plan backfired, horribly.
A new fairy is brought into existence whenever a new concept in the human or fairy world is created, either by humans/fairies or nature.
The Fun Day fairies came about when our modern calendar was created, the Petal Fairies when the first flowers started growing, etc. Humans and fairies have met before.
At some point, before Kirsty and Rachel came, the humans made contact with the fairies. Nothing really special just a concept this troper had in mind for a long time.