I checked the suggested list of Top 50 free educational apps and found that most of the apps listed there are not really suitable for primary-early childhood students. However, they are awesome applications for you as a teacher and for students in upper grade levels.
Checking furhter, I started to find more suitable apps for little ones, such as Drawing Den by Applied Phasor (http://itunes.apple.com/us/app/drawing-den/id343532880?mt=8) and loved it! This particular app is an appropiate tool for younger children and it's lots of funn ! First graders love drawing and coloring and this is a great tool to integrate technology with something they love to do. This tool will also help them to develop motorskills, pay attention to details, and gain confidence with drawing. They can make up stories by saving several drawings to upload later on on Youtube.
I also found Animal Fun by Brian Pfeil (http://itunes.apple.com/us/app/animal-fun/id314278061?mt=8#) which is an incredible tool to teach children vocabulary, both orall and written. It helps students to identify as much as 50 animals by the sounds they make, recall animals' names and learn how to spell their names in english. It seems like a fun thing to do!
You can also find apps for thousands of children audiobooks searching for Free Audiobooks ( http://itunes.apple.com/us/app/free-audiobooks-2-947-classic/id352615816?mt=8 ) and scroll down to the end of the page to find the ones for kids.
BE AWARE that a lot of this apps are not really for free... they have a free kind of "trial" but you need to pay in order to go further than that.