Dreaming Robot Press is a brand new market with a call for submissions to the anthology "2014 Young Explorer's Adventure Guide". The anthology is looking for short stories of 3000 to 6000 words in the various sci-fi genres and aimed at middle grade (9 to 12 year old) readers. The reading period is June 1 to August 31 and payment is 6cents a word. You can find the details here.
A couple of notes here - Please be sure to read all the way through the guidelines. The first reason being that they're going to run a crowd funding and if it fails payment will be delayed. And second, stories will be selected by committee and posted to a private website for reading and voting. While they say this doesn't violate your first electronic rights it's something that you might want to check into just to be on the safe side.