So how did the kids and trolls know what their godtiers were? Like the exact title of it. And why don't they actually know their role? Like, Eridan assumed that being a Prince of Hope was a good thing and demanded to know about all hope related stuff on the meteor.

There are various points at which the Titles are revealed. The first instance is Nanna’s Inscription in John’s copy of the Sassacre Text, in which she explicitly tells John his Session has a Heir of Breath, a Witch of Space, a Seer of Light and a Knight of Time. Then the Consorts talk about their roles, the Salamanders do talk about the Heir of Breath, and the White Queen refers to Rose as the Seer. So, Sprites + Consorts + Exiles tell the Kids what their Role is- But the thing about God Tier is that, you make up your own Role. Two Heirs of Breath won’t be the same, because each of them may consider their Role as a Heir, and their affinity to Breath, in different ways. For Eridan, that mean being Bossy about anything Hope-Related, because he’s an asshole. A Prince of Hope would’ve been great though- Destroying through Hope, if he’d been optimistic and nice about things he could’ve been a good weapon against Bec Noir and Lord English! But in the end he ended up Destroying Hope, and Destroying through the Hope that Bec Noir would become an ally to him. So yeah, you make your role up, you only get vague hints about what the Classes and Aspects mean, and what your Quest is.