"A child is not an adult."
Translation:Mtoto si mtu mzima.
I love how this reads literally. Children aren't WHOLE people. They're just tiny little half things wandering around on their little half legs.
I think when you say Mzima without mtu it may sound that you are questioning the mental ability rather than the physical one.
-zima can also mean "healthy" but to make it mean "adult" it's generally paired with mtu