Do Young monsters or humanoids (or babies) have a different alignment to adult versions of those monsters/humanoids/babies? Do they have an alignment at all? Are they capable of having a non-neutral alignment?
Lacking any exemptions, the normal rules on alignment would apply.
All creatures have an alignment and alignment determines the effectiveness of some spells and magic items. (PFSRD, "Alignment")
It goes on to say this.
Animals and other creatures incapable of moral action are neutral. (PFSRD, "Alignment")
So creatures incapable of moral action are neutral (although some monster entries are mindless yet evil, which I assume is an attempt to indicate the moral considerations of the creatures creating them, rather than the mindless creature itself). However, humanoids are nearly universally considered capable of moral considerations and have alignments. Anything that is a playable character can also have various alignments unless noted in the 'race' entry.
The 'Young' template, the only metric to judge children vs adults in pathfinder, does not specify that the target must be Neutral, or otherwise indicate that the Pathfinder roleplaying game system considers children or adolescents 'incapable of moral action'. The GM can decide that specific children or Young monsters have a different alignment than the norm for that monster or race (or class) in the same way they can alter alignment for any individual npc or monster the party might come across, or in the same way they can alter the alignment of any demographic (poor, mercantile, slaves, wealthy, desperate refugees) that might live differently than the 'normal' monster or humanoid of that type and therefore have an altered alignment. This is not spelled out in the rules, but can be inferred from the general Alignment and Gamemastery sections of the core rulebook.
As user2754 already mentioned also young ones would have an alignment and most of the time a neutral one.
Thus it is easy from a good/neutral/evil perspective. The other axis though is a bit more complicated.
For most children it will be so that they slowly learn rules and to follow them, ... . Thus they (especially the younger ones) do what they want to do.
With that in mind for the Lawful/Neutral/Chaotic axis most younger ones will tend mostly towards the chaotic alignment (only the older children will know enough about the worlds and rules to become neutral or even lawful).
Still though that will be valid only for most humanoids. Some creatures who have a natural tendency towards an alignment will be mostly aligned in that direction even at a young age (or in their initial state of being for some creatures that are more created than born and aging).