I probably shouldn't be shocked, but I am. Firstborn talked with his friend about why a centaur might be a problem, and apparently that's fine: she has other ideas.
Not like, other ideas in that particularly ominous tone of voice, but just like she's perfectly ready to do something else instead. Her second suggestion was a Mousefolk Cleric. Which is a great class, but a homebrew race -- not part of the core books, not playtested in a way I can trust.
I balked a little at first, because the whole point of running the campaign this way was to use core rulebooks so I don't keep having to look things up and/or figure out how things work -- to keep my prep down to something manageable for me, basically. But here's the thing: appearances can be handled in roleplay. If Firstborn's friend wants to roll up a character using, say, standard Halfling or Gnome stats and then roleplay it as someone who looks like an anthropomorphic mouse, what actual difference does that makes? That's right: none.
And gods above and below, I do not want to discourage these kids from flexing their creativity.
But I've gone and looked up the rules for this homebrew race, and it's... well, it's fine. Not only is it not game-breaking, if anything it's a little on the weak side. They're not all that different from halflings in size, stats, and other traits. And y'all, the Mousefolk speak two languages: Common, and Squeak Squeak.
Yeah, Squeak-Squeak is going to be a thing in our campaign. The mousefolk cleric is going to use the mousefolk racial build. I don't even care. The player can keep track of that. I am completely charmed. Plus, we now have a cleric in the party, which is likely to be critical even if it means I have to read up on 5e clerics. It sounds like her background is going to be that a small tribe of mousefolk are trapped in one of the more stable areas of the dungeon, and she somehow got out (probably while one of the adventuring parties was adventuring) and is trying to find her way back and free her people. (Ordinarily, the mousefolk wouldn't be able to leave, for the same reason nobody would be able to enter: the dungeon is only accessible if one of the seven banners is in place.)
This looks good. Ideally, Firstborn's other friend will want to play some sort of tank who can stand up front and hold off enemies while soaking up some damage -- probably a fighter, barbarian, or even a paladin. But if he wants to play something else, I'll compensate with a couple of NPCs. There's plenty of dramatic potential there with possibilities of unspoken secrets, personal motivations, and potential for deception and betrayal. There's really no downside; since if I don't need to add NPCs, or only one NPC, it's less to keep track of; but if I do add more than one NPC, there are more things I can do with them. And I can always adjust the nature of the NPCs to fit the needs of the party, even if it means setting aside the one I'd really like to try out.