I want to have a character whose a child however your character has to be an adult by Pathfinder rules so i am currently looking for a spell that reduces a characters age. I am aware of spells like reincarnation which makes your character a young adult but i am looking for one that makes your character a child. I am looking for something Pathfinder society legal.
Pathfinder Society has strict rules about how effects are handled, which prevent any spell-based strategy from working permanently. In PFS play, non-harmful spell effects cannot last between scenarios:
All spells and effects end at the end of a scenario with the following exceptions: [healing spells, harmful effects, and four specific spells]
That’s pages 25–26 of the Pathfinder Society Roleplaying Guild Guide. Since childhood age is not a harmful spell effect, you’ll lose any age change effect from a spell at the end of every scenario where you manage to acquire it. And since permanency is banned in PFS, there’s no way to avoid losing it.
Furthermore, any permanent childhood effect — from spells or otherwise — would make the character’s age illegal. (PFSRGG, p. 7) Any character that is illegal at the end of a scenario must be reported as dead (p. 23):
An unplayable character should be marked as dead when reporting the session.
So no matter the way you slice it, trying to make a character permanently break the age rules for PFS is a losing proposition.
The only way you could manage it is by having a temporary effect that you could resolve before the end of every scenario — this would let you start and end each scenario as a character of legal age. Acquiring such an ability may be prohibitively difficult though — perhaps impossible within PFS rules.
While there are several spells that can reduce a creature's current age—Blood of the Night (2012) includes the spells steal years et al. that are likely the most straightforward—, few of these spells allow a creature to reduce its age to less than its current age or to less than young adult à la the spell reincarnate.
Nonetheless, the spell cyclic reincarnation—a spell that may be legal in Pathfinder Society play, it coming from Arcane Anthology (2016)—, in part, says, "If the affected creature [i.e. the creature on whom the spell cyclic reincarnation has been cast] isn’t a humanoid, there is a 75% chance that the creature returns to life as a youth of its race, gaining the young creature simple template instead" of the spell's normal effects. Thus a nonhumanoid like an aasimar—an outsider—stands a 75% chance of becoming a young aasimar if it's the subject of a cyclic reincarnation spell. (Make sure someone else at the table has the spell prepared before suiciding and hoping to come back as Kid Angelkin, though!)
Beyond Pathfinder Society play
In this GM's home campaigns, this GM would allow a player who's adamant about playing a creature made young this way to write in the PC's background that the PC was the subject of the spell cyclic reincarnation before entering the campaign.
However, typically, in my campaigns, if a player wants his PC to be younger than the minimum age recommended by the core rules, this GM simply says, "Sure. Whatever." The game is far more unbalanced by old PCs, youth granting a PC no particular mechanical advantages.
Note: This GM has personal misgivings about players playing actual little children in a role-playing game that has as its mission statement kill things and take their stuff. Just sayin'.
The simple answer is there are no spells that can turn you specifically into a child. Just because it isn't written in one of the books specifically should not stop you. You just have to get a little creative. You may modify some spells reasonably enough to make this possible. The spells I'm about to suggest will require your GM to allow for some modification of the spells.
The spell closest to what you're looking for is Threefold Aspect. At its basest description, this spell ages you younger or older into a young adult, an adult, or an elderly adult. This changes your Ability Scores a bit for each form. You can try consulting your GM to see if you can add "child" to the list and use the Young Character stat modifications for this effect.
Or, instead of that spell, you could request your GM to allow you to turn the target of Bestow Curse or Greater Bestow Curse into a child using when your character casts it as an alternate effect. I don't particularly recommend cursing yourself though, as it is a permanent effect until otherwise removed by magical means.