A comma splice is a common grammatical error in English. Writers most often make this mistake when they are trying to "write by ear." It's a common idea that a comma indicates a pause where a reader or speaker should take a breath, but simply adding commas when you feel a break is needed is not a reliable way to make sure you're punctuating your sentences correctly.
A possessive noun quiz is very useful for fourth and fifth graders. It's especially helpful in teaching about plural possessive nouns. Below are a few sample quizzes that you could make use of when teaching grammar lessons on possessive nouns. Each quiz is available as a downloadable PDF, which also contains the corresponding answer key.