Child/teen psychology source

The best resource is kids themselves. Sit quietly in a mall and look for tweens. Listen in. Do the same at parks and other spots where they hang out. Try not to appear creepy.

Also, read and watch material geared toward your audience. See what they are into.

Look for books on child development and the tween years. It's not so much psychology that you need.

And listen to Sage--YA is for kids over twelve and the protagonists are in the late teens. Middle grade is for ages 8-12 and the protagonists are at the upper end of that range.

Get to know kids that age. A lot of successful children's writers are teachers or former teachers - they have a lot of experience with kids. There is no substitute for this. You don't have to be a teacher or have a job that involves working with kids. Being a parent or spending time with little cousins, nephews, nieces, friends' kids etc or similar is also good.

Also, remember that kids are as individual as adults - and we were all kids once (don't forget that you, remembering how you were at that age, are an excellent resource). For younger children, it's useful to know what age kids hit various developmental milestones (and to remember that these are averages and there's a lot of variation) so you don't have kids doing things that are completely developmentally inappropriate, but there's no psychology or child development book that's going to tell you how to write children, but this applies more for younger children than for tweens and teens.

If you find yourself asking "would a child aged 12 do xxxx?" "would a child aged 12 like yyyy?" etc then that's going about it the wrong way. Writing adult characters, you don't think "would an adult do xxx" - you'd develop your adult character's personality and background as a unique individual. Same with kids.

At age 12 most kids can do all the same things the average adult can do if they're taught how and the skill doesn't require several years at university to learn, but they lack life experience and they are prone to fits of immature behaviour (they can revert from all grown up to childish in a matter of seconds, but probably not in public). The adolescent brain goes through a stage of neurological development that results in them needing more sleep and being more prone to impulsive behaviour and bad decisions, then combine that with a total lack of life experience... but at the same time they're also very capable and can, if needed, take on adult responsibilities and do adult tasks, as long as they've had the opportunity to learn how. Adults tend to grossly underestimate kids.

Also, I've said it already but it needs saying twice, kids are as individual as adults. Some 12 yr olds will be much better at the above than others, just like some adults are. (And not all 12 year olds are adolescent yet - there's a lot of variation regarding when adolescence starts and boys tend to enter adolescence a couple of years later than girls, on average.) Develop the characters the same way you would adult characters but factor in adolescent brains and a lack of life experience.