@Symbeline says:
In most cases both the dinosaur and dog would vouvoyer the one they are talking to, and the other way around as well. This rule (pretty much based on mannerisms) is allowed to change if the dinosaur and boy know one another personally. Then they can tutoyer. Say the dinosaur always knew the boy but the boy didn’t, out of respect, the dinosaur would vouvoie him anyway. (for whatever reason that only one would know the other, not important)
Mostly you only tutoie people if both know each other, like a couple, close friends, siblings.
The puppy and small boy can be forgiven because both are young, so they can tutoyer. (zomg trying to insert proper French verbiage into the English language ow my brain lol) You know, they’re both young and innocent, so it’s cute, not impolite.
So if I stay with your children’s book idea, most children’s books will say vous and not toi, regardless of just about what’s going on in the book or who is speaking, since it’s part of the culture’s traditional importance on being all snobby and polite. XD I mean for France, although I hear Belgium is much the same.
Québec is the exception. Although all French books here are pretty much like ones from France, people here rarely vouvoie one another unless it’s something like a job interview, or meeting your boyfriend’s parents. And even then.
But I hope this helps. :)