One of the best (arguably THE best) things about Bionicle was its self-conistent and non-jokey universe. I think introducing human, especially in this manner, to the Bionicle world would undermine that consistency. Bionicle has always pretty clearly existed in its own, distinct universe. Introducing human characters alongside Toa and Matoran would be ridiculous on multiple levels. Most obviously, putting humans alongside super-advanced magic robots would be silly on the face of it--introducing humans automatically makes a story seem more realistic; it's easier to accept fantastic elements if no humans are present. That's not to say that humans and fantastic elements can't co-exist, but if you're telling a fairly serious story, you have to go a bit further out of your way to justify the unbelievable elements if humans are involved. And that would be difficult with Bionicle--there's no conservation of energy, we hadn't seen any purely biological life at that point, so introducing human would've raised questions about how such life came into existence, and at the time Bionicle had a strong mystical/spiritual theme that would've been problematic to explain in human terms. And that's just the tip of the iceberg.
Beyond this, it's unlikely a human child character would have been handled well. A human Takanuva would've almost certainly been intended as an audience stand-in character. After a couple of years of having no such character (at least not in such a direct manner--you could make an argument that Takua as meant as an audience stand-in), it would be strange to have one. He would likely have ended up asking lots of questions about things we already knew in order to dumb things down for casual fans. It's also likely that having a kid save the island would change Bionicle's tone from an epic adventure to something akin to one of those corny Karate Kid sequels.
Lastly, Takua turning into a human and saving the day would suggest that humans are superior to Toa and Matoran, thus invoking Fantastic Racism.