Thule’s voice, well, I think it must be something not so far off from Mu’s. Velvety soft, eloquently spoken and with resonant low tones, perhaps a bit more than Mu’s. Maybe something like Sho Hayami (on his higher range).
As for the kids being all spunky, I will say I fear that’s not the case. I believe it depends on the temper and education of each child. In Kiki’s case, for example, I headcanon his first years he was raised in the countryside before they turned him to Mu for tutelage. His use of the pronoun Oira hints of that. It’s his admiration for Mu that makes him try to emulate him in a more formal speech and poise as he grows up.
In Mu’s case, his character being more mellow, I believe he picked up Shion’s Pope-like official and elegant forms of speech from a very young age since I headcanon he was raised inside Sanctuary.
And for the rest of kids in the sheepverse, all of them are different: some very well behaved, some just really energetic, and others that are a complete handful xD
What is culturally a rule is the respect of life, arts and knowledge, so they all need to have those traits as adults in some way :)
