Hi Rachel, I very recently happened to get my hands on “Simply Grammar, An Illustrated Primer”, by Karen Andreola, at a used book sale our local HS group was hosting. I think this is the only volume, so likely this is the one you are asking about (?)
I haven’t started using it yet as my kids are all still young (I bought it for down the road and really just picked it up because it was so reasonably priced at our HS group sale…) but after seeing your post, I checked the introduction by Karen Andreola. She suggests the book can be used starting at around age 9, though she also mentions that it is usable as a reference book even up through high school for core, basic grammar.
As I leaf through the pages, many of the lessons seem quite simple in the early part of the book. Very nice content, but not challenging. Hopefully it would serve your purpose – keeping them fresh on grammar skills — but you should know, the beginning appears to be truly entry level grammar. I’m sure by the mid-point of the book, you’d be fine. And the pace recommended is only once per week or so. With this, even if you used only half the book, it should do ok for filling in your gap period.
Hope this helps a little!
Andi