pathfinder beastiary pg 296
Young Creature (CR –1)
Creatures with the young template are immature
specimens of the base creature. You can also use this
simple template to easily create a smaller variant of a
monster. This template cannot be applied to creatures
that increase in power through aging or feeding (such as
dragons or barghests) or creatures that are Fine-sized.
Quick Rules: +2 to all Dex-based rolls, –2 to all other
rolls, –2 hp/HD.
Rebuild Rules: Size decrease by one category; AC reduce
natural armor by –2 (minimum +0); Attacks decrease
damage dice by 1 step; Ability Scores –4 Strength, –4 Con,
+4 size bonus to Dex.i can use this but it dont change the HDs is there a other or batter one to use?
–2 hp/HD. not hps HDs!
The game does not account for killing baby monsters. I would just reduce the HD until the monster was easy enough to be where you want it to be. Some monsters might even be defenseless at a young enough age so stats won't really matter that much.
Baby goblins or w/e aren't usually a threat, except possibly to your alignment. Seeing as they aren't a threat, they aren't worth experience and don't need stats.
If the newborns of some species are still powerful enough to be a threat (baby tarrasque!), you can lessen its power more than the Young template does (probably by taking off HD). There isn't a template for this sort of thing, though, you'd have to do it by hand.
You can also use the Advance Monster rules and work backwards to create a smaller, lower HD creature.