We've really enjoyed the books in the Eagle-Eye Ernie series by Susan Pearson and Gioia Fiammenghi. The stories are a bit old, but they are fun and entertaining. This story might confuse kids today a bit because of the reference to the tapping noise a typewriter makes, but I suppose you could somewhat equate the noise to the softer tapping of a computer keyboard. In any case, it's a short book and an interesting mystery.
Suffers from obvious plot and a story that's been redone to death, but for kids they would eat this up. For small children the suspense for the small group and the different clues, as well as solving the mystery as they go along, would be enjoyable and age appropriate.