oooh! A Series of Unfortuante Events... I tell ya, the author does have an Edward Gorey sense of humor. What I liked about them was that if he used a large or unusual word, he'd always let the reader know what it means. And you do get sort of caught up in the series because you really do root for the children and hope that they have a happy ending in at least one book, but because it's a 'series', no luck.
I think that it is age appropriate for your cousin and second grader. In the bookstores they are displayed around that 'young reader' type of age group. Pick up the first one yourself and see what you think. You might think it too deep or even too sad. I mean they are orphans, ya know?
Another good series are the Dear America series which are in a diary type format. Your cousin might like "Detectives in Togas" I can't remember the author, but he does a bunch of those history series types. Suppposed to be funny.
Sorry this went on too long! I work in a bookstore and kids books are my favorite subject!!
Take care!