To spice up a chapter of Rise of the Runelords, I want to introduce an additional NPC: human child (about 8 to 12 years old). The child should be saved by the PCs and brought back to her parents. What would be good values for armor class, HP etc? I could not find good informations in the Core or Game Mastery guide.
Use the Young Template
Young, a simple template, is pretty much exactly what you're wanting. Take an NPC class - I suggest Expert or maybe just a single humanoid hit die - and then apply the template to the resulting creature. Voila, instant kid.
There are two ways to deal with this, the first is to follow an old adage in RPGs that goes something like this:
If you provide stats the PCs will kill it
You don't need stats for the child NPC because the PCs should avoid enterign combat with the child at all costs, after all they are trying to protect the child, not get it killed. Even when they are defending the child there is no need for stats as there are only three possible outcomes if an enemy gets close enough to execute an attack against the child - fatality, wounding, or a lucky escape. it's much better that the GM determines that outcome, not random dice and strict rules.
In this way you can establish drama through description without delving into the details. Getting hung up on the mechanics is one of the classic mistakes you can make as a GM. Your players generally don't want or need everything nailed down with statistics, what they want is to enjoy the adventures of their characters.
The other alternative, for GMs who prefer to have everything prepared up front, is to treat the child as a fully fledged adult NPC with a minor modification to their stats. e.g. -2 STR, or, as pointed out elsewhere, apply the Young template from Pathfinder.
This second approach, while adhering to the rules as designed, can lack flavour, and if the dice gods decide against the players, make for a bitter outcome to a potentially awesome evening of play.
Of course, your mileage may vary.
There was something posted by Rich Burlew on the GitP forums regarding baby dragons that I think applies here nicely. After all, if it's a normal human child, it's not combat-trained, it's not especially tough, it's not going to do much (if any) damage barring certain situations, and getting them killed in the crossfire is going to be an issue that I'd personally rather just handwave.
Movement: Gets away if you let it.
Saving Throws: Miraculously survives all accidents.
Armor Class: You hit.
Hit Points: Congratulations, Baby-Killer.
Special Qualities: I hope you can live with yourself.
Coincidentally, these are the same exact stats for every other species of baby.
Believe it or not, there are rules for this! The most important sections are quoted below.
Attributes and Skills
A young character has a +2 bonus to Dexterity and a –2 penalty to Strength, Constitution, and Wisdom. (A young character's potential inexperience and awkwardness are represented by having only the skill ranks of a 1st-level character rather than taking a penalty to Intelligence or Charisma.
You can select only NPC classes while in this age category, beginning play and advancing in level as an adept, aristocrat, commoner, expert, or warrior, according to your interests and social background.
Most of the rest of the rules deal with issues related to playing young characters, roleplay concerns, and transitioning to adulthood.