Apps allow non-verbal children to "speak"
"Applications like Touchchat allow non-verbal children, such as those living with Cerebral Palsy or Down Syndrome, to select images, words and phrases with the tap of a finger and "speak" with the help of the device's speakers."
See below for some apps in this category. (The fully featured ones can be quite expensive for an iOS app.)
More suggestions on "Autism Speaks" website