Why don't we want to talk about grammar?
English speakers are notorious for not having a very strong understanding of English grammar. Many of us who are of teaching age were educated at a time when ideas about language development did not include, and often explicitly excluded, focus on form. Not knowing the syntax and structures of your own language makes it harder to learn other languages in many ways; contrastive awareness only works if you can compare a form in a new language to the same or similar in your own language. The terms we use to describe the components of a language are a critical element of language awareness. Being able to identify something as an adjective or an adverb gives us a common metalanguage to use both mentally and in learning situations. Despite this, I often hear teachers (and others) putting forth arguments against using "grammar terms" or "teaching grammar" to children.
In large part, I think this can be blamed on our own insecurity about grammar - if we didn't learn it growing up we may never actually have learned that metalanguage unless we studied other languages. I remember clearly an example from my first "Pedagogical Grammar" class, when I started my BEd. The teacher put a long sentence on the board and asked for a volunteer to parse it. None of the English native speakers had a clue what she wanted, but the one French student went up to the board and parsed the sentence quickly and accurately. She was the only one that knew what "parse" meant, and she was also the only one who knew the correct terms for the parts of the sentence, from the simple (subject, verb) to the complex (conjunction). That was when I realised I had some serious learning to do before I could teach English!
My response to the claim that children don't need to use grammatical terms in order to master language is this: in all subjects, we have technical terms that are used to talk about the subject accurately. When teaching math, we would never teach the children "If you put two and two together you get four!". We use the correct math terms "plus" and "equals" to create a common language to talk about specific functions. The equivalent to math function words when teaching English is the words we use to talk specifically about form and function; grammar words. By introducing these terms at age-appropriate points, we give the students agency to discuss language in ways that are not possible without metalanguage. This is powerful for all children, but especially for language learners, who can then use those named forms or functions to compare to their own languages, and gain better insight into how English is similar/different from how they already understand language to work.
When we say "every teacher is a language teacher", and encourage teachers to plan for language development throughout the curriculum, this knowledge about how English works and how to describe it is critical. Without it, students are constantly hearing teachers use different terms for the same things, or terms that lack specificity for contrastive awareness (Wow words, anyone?). So for all of us who were raised in a grammerless wasteland of "whole language" teaching, it's time to hit the books so we can be an expert in our subject - being English teachers in our multilingual classes. Being fluent in grammar-talk isn't any more difficult than learning to use maths terms or science terms, and we've all done that!