What is with JP authors vastly overestimating the language abilities of kids? 5-6 you can start having real but somewhat simplified conversations with them, and typically most young MCs are already way more eloquent than that. At 3 little bro here should barely be stringing together sentences.
I think if I met a 3 year old that could speak like this guy on page 9, I would start running lmao.
Isekai fantasy is basically the lowest par with the highest demand in the web and light novel scene in Japan. If you need to start with one kind of literature to launch your writing career, then that's it. In middle school or high school. Very little background research absolutely needed, as is evident in most isekai. If you start to write a hobby or sports related novel (or manga directly), for example, you need to know what you are writing about or the audience will call you out harshly. But if you write fantasy, you only need to know how to write.
If some high school age or slightly older boy/young man, with no relationship experience, writes fantasy, it makes sense they don't really know what a little kid should realistically be like, unless they just happen to be from a really big family or have lots of younger relatives they regularly interact with. But Shinzo Abe was assassinated for desperately trying to encourage the Japanese to produce more offspring, so it's extremely unlikely the author would have such circumstances. So, they simply don't know what kids are like and how they behave.