Providing detailed descriptions can be a challenging task for language impaired children. In fact, describing can even be difficult for children who do not have language impairments!
In order to provide good descriptions, children need to explain attributes such as object function (what objects are used for), where certain items may be found, and what category the item falls into. Children also need to be able to tell what makes a certain item special, or different, from other similar items. For example, a child might tell you that an apple has a core and a stem or that a tiger has stripes.
In addition to have difficulty with describing objects or pictures, children with language impairments frequently have trouble describing what people look like. Therefore, it is often helpful to encourage children to describe people that they know well before attempting to get them to describe strangers. Encourage the child to pay attention to the clothes people are wearing as well as paying attention to facial features and body characteristics.
There are many games, books, and activities that can help children improve describing skills. I have listed my favorites on this site for your convenience.