I also have two little boys, one of whom may be an INFJ. At any rate, I am sure he is I, and suspect he may be F also. He is very concerned about privacy, and would be hurt by some of the behavior described above. (Who wouldn't! That's awful.)
My other son is very E; you can practically watch his energy zoom up when he is around people. He is very sweet and sensitive and outgoing, and likes to play with hair, (mine. all the time) and he wore my pink hair band to school the other day. The INFJ got embarassed and he is the one who told him to take it off, and he's the one who gets embarassed easier. The EXFX seems to be immune to embarassment, whether by age or temperament, I don't know. The INFJ also criticizes his little brother for saying "Hi" to literally everyone we pass because it embarasses him.
My INFJ is shy, but not painfully so. Shyness, not Intorversion, is what sucks so very much in school! Believe me, I know!
You can help children overcome shyness with practice. My INFJ always bonds with one or two other children, and if he likes his teacher (which he does, this year, thank goodness) he is happy, and an absolutely stellar student. You've got to watch it if he doesn't like the teacher. You'll KNOW if he's happy, it's super-easy to tell, and if he seems unhappy, you'll know there is a problem and then you can figure out what to do to help.
We've enrolled my INFJ in drum lessons, and it's really good for him emotionally and mentally. Also we try to just be very accepting -- and it helps that my husband is also sensitive and gentle. He is not one of those emotionally repressed macho jerks that enjoys humiliating others. Yuck. So it helps to have a good, stong male role model! A LOT.
I will say my INFJ is perfectly capable of dishing stuff out also, and can go from 0 to berserk in about half a second. I'm talking rage. So obviously, we keep an eye on his peers, teachers and environment to make sure he is not being provoked or teased, and talk to him about appropriate expressions of anger.
Schools here are VERY on top of bullying; almost to the point it is borderline ridiculous sometimes, to my adult eyes. But it is better than the alternative. I'm sure my son would be quite capable of hurting someone severely given enough provocation.
He's also incredibly protective of others, including me (unnecessarily!) and has hesitated to tell me things (like what football team he likes) because he thinks it will hurt my feelings (not even a little bit). I have to tell him that mommy is perfectly okay with him rooting for whatever football team he wants, I could not care less about football and will buy him whatever kind of jersey he wants, no hard feelings whatsoever. But he was really worried about that! So I have to be careful to let him know I approve of him, all the time, and we're neither of us harshly critical. Encourage him to talk and don't embarass him! Especially for having emotions.
The EXFX is not at all shy, lucky child, he just rolls with things, he doesn't care! Easygoing little guy. He's also already proposed marriage to about 5 girls, and despite the hairband, will not hesitate to tackle people who bother him, he's very direct and physical. Then he gets over it just as quickly.
At kindergarten age, I don't think the children exert too much peer pressure to hold in your feelings or not express yourself -- usually -- it kind of depends on where you go to school. My friend does a homeschool group and it is quite frankly sort of hippieish (in a good way) and it encourages emotional expression much more than is typically found in a mainstream environment.
There are gender roles in Kindergarten (the hair band did not go over too well, actually...) but there are PLENTY of sweet, expressive, feeling little boys. Lots of them. Love 'm to death, too! <3 I help out in the class fairly often and can vouch for that. Hugging, being sensitive, etc. is no problem at all for boys in Kindergarten. Middle school? ehhhhhh.... maybe then it might cause some problems, we're not there yet.