Writing children is hard.
At the very least, it is for me. And I'd wager it's hard for a whole lot of other people too, because a lot of kids in fiction are... not so good (especially when they're written alongside adults.) They either sound like "basically an adult and the age is just cosmetics," or "reduced to a cute and stupid lamp post."
In Stars and Time fucking nail Bonnie though, and I think the secret to that is that everyone always takes Bonnie seriously. If Bonnie says "this sadness has a weird smell," Odile responds with "ok, what other things have notable smells to you?" If Bonnie says it's snack time, well, snacks are their trade, we can only bow to it.
Bonnie isn't a child character that is small and cute and stupid; they have their own opinions on things, their own skillset, and everyone accepts that. What differs between Bonnie and the adults is the experience; being so young, Bonnie often lacks the words to articulate their thoughts accurately or tactfully. (Also, lack of life experience means there's a lot of adult stuff they don't know/only make assumptions about, hence why adults might clown on them by making them drink water pretending it's vodka.)
Another thing is that when writing child characters, a lot of people want to... let's say, shield them from horrible things. Which often ends up in everyone dying without the kid ever truly being in danger, and/or the kid being too "innocent" to understand death. But kids DO know what death is! And pain, and grief! Maybe they don't fully get it, but aside from very young children, most kids understand the concept of "sometimes people are no longer here and you'll never see them again." Because, well, a lot of kids do go through little tragedies through their lives, even if it's something as simple as "the family pet cat died."
And Bonnie, through ISAT, is written as someone who is painfully aware of death. It's not at the forefront of their mind 24/7, and when they DO think of death they sure don't know how to handle the emotional side of thing, but they know death exists. They know it's a very real threat for their friends. For their sister. For Siffrin, when homie lost that eye. Bonnie is treated as a part of this world, not some add-on who didn't get the grief dlc.
It's very refreshing to see! When the writing is good bottom text.