"Bitsboard" is a free app that can be downloaded on the ipad and used for Auditory Bombardment of the the target sound in the initial position of words.
You can just browse the catalog for "phonics" and download the "CH" initial board. This app allows your child to practice listening to the "CH" sound as they play games. Some of the games available are shown below, along with a picture of the pictures they will be using for this activity.
Books that can be used during Auditory Bombardment of the "ch" sound: