I used to teach Spanish. I discovered that the only hint, whiff or slight inkling of grammar that most of my students had was when they found out about it from learning Spanish. Oh well, it was an opportunity to teach them English grammar for comparison purposes.
That was one of the great purposes for teaching Latin, back when good schools routinely had a Latin requirement. Lots of grammar. It was intended to carry over to English — and of course they taught English grammar too, back in the day. The ability to compare English grammar with Latin grammar helped on both topics.
But they largely dropped Latin. And they largely dropped English grammar. Spanish may be about the best chance a kid has to find out that languages actually have rules.