To Josh31: good point. Also, by putting this pressure to conform on the kids at such a young age, we are putting them in a terrible position if they need some breathing room that is well within the parameters of healthy Jewish living. At that point, they see it as all or nothing; if they want to wear a striped or knit shirt, or wear – gasp – khaki pants, even if a dark, conservative, cut vs. wool or a blend, they may feel that they may as well do who knows what else.
I have nothing against white shirts after bar mitzvah, but it should be totally voluntary. Our kids have to know that at whatever age, if they walk into a shul in clean clothes, tucked in, laces tied, in a way that is b’kovodik, Hashem will listen just as well.