I teach preschool special education and I have used various assistive tech. devices with my students. Voice output devices are often used with children who have little or no communication. These can be as simple as a "big mack" button (one button that can be programmed with a phrase or word to use) the child can then push the button to convey what he/she needs... example "I want more" or a voice output could have many different buttons, be computerized etc. The Boardmaker computer program is invaluable for pic. communication/prompts.
My children also use switches and switch activated toys. Generally, these are used to increase children's understanding of cause and effect and are used with children with limited mobility. These include an access button or switch (I have used hand activated as well as head activated switches) and toys that will then activate when the button is bushed.
There are some WONDERFUL web sites that will give you an idea of different assistive tech. devices and how/why you might need to use them. If you go to my blog www.howlongisthishall.blogspot.com I have put together a page of "Links I Love" There are many different links to sites that focus on various items for children with special needs.
Hope this helps!