Not only Tama is a little girl, she also happens to be the victim of a tyrant that took over her island and made the life of everyone there a living hell and is trying to fight back the way she can. Ring a bell ? Who happened to be in that situation? Geez I wonder why Nami would feel it extra hard'
Here Nami losing all inhibition and talking back to a 300 millions bounty enemy with Usopp even telling her to calm down and not do anything stupid. That happened 350 chapters ago.
The point of Nami and Usopp is that they're cowardly characters, but like the rest of the Straw Hats, they're courageous as fuck when pushed to their limits. For Nami, it's children, just like Punk Hazard. For Chopper, it's using medicine to do evil, for Luffy, it's hurting his friends or treating you own underlings like shit. I can go on