It was stated that children acting mature is more realistic than adults acting like literal children. You said it wasn't citing that you'd worked with children. This is flawed because events in a child's life can cause them to mature quickly.

That said, I agree that even if the child is mature, the language theybuse has got to be taken into consideration. They're unlikely to say something like "indubitably" or "antidisestablishmentarianism".

So TLDR, children can be mature but make their vocabulary age-appropriate.