As #2 establishes, it is all about the underdog. Matilda is a character who has everything going against her, yet somehow manages to rise up above impossible odds to achieve that happy ending. The way that she is able to use her intelligence and special power to bring justice to the "baddies" of the story, one of whom is perhaps the most terrible teacher I have ever come across in literature, is something that we all enjoy.
In my opinion, the novel Matilda is a favorite among examples of Children's Literature because it features a spunky character sporting a phenomenal super power and surpassing powerful odds. We, as readers, are given someone to root for as little, sweet Matilda is brought up by her horrid parents and terrible principal. We enjoy the use of her super power (her ability to make objects and sometimes people move at will) as it improves her situation. We also rejoice greatly in her relationship with the dear Miss Honey and the fact that she has such a wonderful part in the happy ending to this book.
MY opinion is that kids like it because it is funny and educational at the same time. also agreeing with you ms.charelston because it has powers in it that most kids wish they could do if they had powers