Four children and It

In this book there is four children. One child is reading a book and finds herself and three other children inside the book.

Characters:

1. Rosalind:  She is the oldest ( though sometimes she wishes she wasn't).
2. Robbie: He is Rosalind younger brother.
3. Smash: She is their stepsister and she isn't too happy about it.
4. Maudie: she is the baby of the family- everybody loves Maudie.

Now you've met the four children, but what is IT?