It bothers me how much fans seem to think a child audience would hurt a BIONICLE TV show. The Mata Nui Online Game was one of the absolute best methods of story delivery in the entire course of the theme, and I don't think there's anything in that theme that would push its rating above TV-Y7. It had some extremely energetic fight scenes (Onua vs. Lewa, or Kopaka vs. the Muaka), some really dark and foreboding moments (the entire Po-Koro chapter), and even some really powerful philosophical concepts (various monologues from the Matoran and Turaga, or Makuta's monologue before the final battle). It also had a fair share of humor (Taipu's slow-wittedness, Hafu's boastfulness, Macku's endearing relationship with Hewkii, various background gags, and the very concept of a "Taxi Crab").
I take issue with the idea that darker and edgier storylines are somehow "better" or more legitimate than more lighthearted fare. Most superhero cartoons are rated TV-Y7, but a lot of them still have complex storylines and characterization that really help to make the characters relatable and the stories memorable. Some storylines even deal with very serious issues, like Avatar: The Last Airbender's themes regarding terrorism and genocide or Static Shock's frequent focus on societal issues like gang violence.
Keep in mind that BIONICLE was primarily for kids from the very beginning. Even the more violent moments in the books were no different: it's just that young kids and adult censors alike have a stronger stomach for violence in written form than in animated visual form. Thus, any BIONICLE media that seeks to shut out that child audience is forsaking the true spirit of the BIONICLE theme to instead embrace some distorted vision of what the theme should have been.
Source: Bionicle TV Show