Many children (and quite a few adults) don’t think of air as matter. It’s invisible, seemingly immune to gravity, has no taste, makes no sound. When you light a match, it burns up and disappears into “thin air.”
This is a problem.
The stuff of matter, the stuff of stuff, seems simple--we mostly rush through it in science class, assuming everyone knows what “matter” is, because, well, it's so simple, and then we expect students to grasp all kinds of nonsense labeled “science.”
The typical school definition of matter is "any substance which has mass and occupies space," a deceptively complex answer. Most students equate matter (or "stuff") with mass, and with it lose any chance of truly appreciating the physical sciences. (Oh, they'll muddle through using algorithms, and such, might even ace an introductory physics course, but they won’t touch the physics again.)
Mass is the quality of stuff that resists change. (More precisely, mass is the measure of inertia in stuff, but I'll leave that be for the moment.) How do we know something has mass? If you push it, it pushes back.*
This is a big deal. Inertia is a huge concept, really the whole shebang of introductory Newtonian physics, and ultimately the basis of the interesting bits of classic chemistry and biology. Inertia is what makes mass mass, and without mass, we have no physical universe. (The “take up space” part of matter only makes sense if you grasp what mass is—otherwise, it’s superfluous.)
How much time did you spend on this as a student? As a teacher?
Let’s go back to a child—how can a 7 year old grasp what matter means (or whatever word you care to mean for mass)? Forget the word mass for the moment—let’s make it a more interesting question. What makes stuff “stuff”? This becomes child’s play.
The conversation can wander all over the place. Do you have to be able to see it? How small can it be? Is air stuff? What’s not stuff?
Does a class have to arrive at a textbook definition of matter? Of course not, not in 2nd grade (or any grade, for that matter). The problem with the textbook definition is that the goal becomes learning the definition instead of learning science.
If a 2nd grade teacher does not feel comfortable discussing matter, then discuss “stuff”—you will wander all over the place, and if done right, learn about looking at the world. Don’t fret so much about not getting to the definition—what we’re doing now leads to the ignorance of certainty that keeps astrology and homeopathy alive. Is air stuff?
Learning science and memorizing definitions are not mutually exclusive. If the goal of a lesson becomes the definition, though, you lose the science. The problem is exacerbated by the concept of “a lesson”—science cannot be broken down into prescribed chunks of time. Traditional lesson plans are deadly to science education.
*Newton’s 3rd law, of course—it’s a big deal.