Reserve this for an older kid (10+?) reading below grade level, as a book that though relatively simple to read will hold an older kid's interest.
The book is well written, with a quite clever, very well-executed premise (of going deeper into into a receding series of paintings), and with numerous good illustrations adding to the story.
But I do not recommend it for younger kids. Emily leaves her 4 year old brother alone in the woods next to a stream -- no water safety message there.
And I am wary of any fire-setting theme in a book for young kids: here the ghost girl is described as having died in what was perhaps a self-set fire, and Emily's 4 year old brother has a habit of going around with matches and their eventual escape is when he (the 4 year old) sets a fire! (Which, by the way, is a clever resolution of their predicament -- I just don't like a positive result to fire-setting by a 4 year old!) And overall, the ghosts are sad and a bit malicious; that faint air of menace will appeal to (especially older) kids, but I don't care for menacing ghosts with younger readers.