Try out the MathEdge Addition Lite and determine if this app will work with your classroom environment.
If you choose Practice, you then select which level of practice: Easy which is single digit addition, Medium which is double digit without carrying, and Hard which is double digit with carrying. This last set is perfect for practicing problems that require you to carry. The example below shows how the app carries the tens digit up to the top after typing in 16 as the answer to 7 + 9, and awaiting to the user to type in 10 as the answer to 1+3+6.
If you enable the volume, students will be prompted how to work through the problems in all practice environments.
Try out the app, and if it works for you, MathEdge Addition/Subtraction app is $2.99. There are corresponding multiplication and division apps.