Yeah every piece of armor or weapon has a weight value and you can only carry so much weight. Kindof like Oblivion. Since kids dont have a weapon to carry, they have extra weight they can carry.

It depends on the class and your equipment, To be honest staying a kid as a martial artist would require excellent defensive and con equipment to make up for your shortcomings pvp wise, but the idea has merit. It allows you to be far more versatile in your skill build, with debuffs and such included as you have the points to spare.
Same goes for Namekian or Maijin kid. Both of them allow for excellent capabilities if u intend on staying permanent kid, but personally id only attempt Perm Kid on Mighty Maijin and Martial artist. they both have skills that are powerful even if they go onto masterclass, and are easier to equip and level besides. But thats just me