Charlotte didn’t focus on parts of speech until the student was at least 10 years old. I figure 5th or 6th grade works, depending on the student.
Yes, JAG is designed for students (and moms) who are just starting with grammar. It’s very easy to use, and the instructions and notes are short, to the point, and conversational. You might want to take a look at the JAG sample pages on the Analytical Grammar site. They are for Unit 4, so don’t panic. The first three units walk you through nouns, articles, adjectives, and pronouns. But the sample lesson will give you a feel for how Robin explains things. And if your kids like math, they might like the diagramming aspect because it’s a lot like engineering (analyze the parts and piece them together). There are also samples from the teacher book, which contains all the answers.
It’s really quite concise and clear.