Older kids might find Khan Academy to be a more useful resource (and free), but for Primary School aged children particularly, this is pretty good and used by a bunch of schools. It's also a great tool to use during school holidays over Christmas to make sure kids retain the knowledge between school years, often lost from lack of rehearsal.
$99 for the first student, discounts stack for additional students.
Our local public primary uses this and provides the logins to students.