This year, Highlights for Children is soliciting mystery stories for their annual fiction contest. They will award $1,000 to the winning entry. (ETA: they will choose THREE winners.)
Stories may be up to 750 words long and must be postmarked between January 1 and January 31 2015. Three winners will be announced in June and all other entries will be considered for purchase/publication.
For all the nitty-gritty details, you can read the guidelines by visiting the Highlights Fiction Contest page.
Good luck and happy writing!