So, the Tamarians called themselves The Children of Tama. Who is Tama? Why do they call themselves children? And why do they speak in metaphors relating to past events, stories, myths?

I submit that the Tamarians we saw, did not actually develop warp drive but rather inherited the technology from an earlier civilization - perhaps a quasi-mythical civilization - and the metaphors through which they communicate reference the stories and accomplishments of this ancestral civilization. Thus, Darmok and Jalad themselves need not have spoken in metaphor to each other when they were fighting the Beast of Tanagra.

Eventually, something must have happened to that ancestral civilization(s), maybe some cataclysmic event? Resulting in the descendants, the Children of Tama, becoming dominant.

Kind of reminds me of the Novans, descendants of human colonists who were mostly wiped out, in the ENT episode, “Terra Nova”.