Adding to that, there should be a distinction not only in singular vs. plural for a(n)/some, but also countable and uncountable. For example, nobody (unless if one chooses to be strange) ever asks, "Do you want 20 spaghetti noodles?" Instead, it's considered uncountable, even though it's possible for one to count. Or for cake, one doesn't a cake, but some cake (or a piece of cake).
Here is a modified ppt from BarryFunEnglish I used for 5th grade last year. The book doesn't offer this distinction in their curriculum, and they should.
Edit: Here is a worksheet to test their ability to distinguish when to use a/an or some. You can do it in class, assign as homework, or use it as a quiz.