Chances are, no one will sue a peewee team and their parents for trademark infringement of a professional team's trademark, but nonetheless, although on a small scale, it's still trademark infringement. To avoid doing that, you need to change so much, that an observor can no longer tell what MLB team you're trying to associate with your use. There's no formular or percentage, it's just a matter of not being allowed to trade on a trademark that someone else owns. If your use causes "consumer confusion," then you've committed trademark infringement, because you've confused consumers into thinking that the MLB is the source of your products and are stealing sales from them, which will make them sue you.
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Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
While Major League Baseball Properties will not likely sue the coaches and parents of the little league players it will sue those who make the uniforms for them. See http://j.mp/d55vNh and http://j.mp/aY96y0 and http://j.mp/bZMmwm . What you're doing is admirable but just be sure you know what you're getting into ahead of time. Good luck.