Your kids already know about Sparknotes. They are just like Cliff Notes except much hipper, and without those telltale caution stripes… and in the case of the online versions, absolutely free.
There’s a lot more than literary guides at Sparknotes.com, though, including a ton of test prep materials and lots of college selection and financial aid help for students and their parents. Here’s a good link for senior parents to start: SparkCollege: Find Your College. Find it Fast.