Expanding their sentence length is definitely something that will help. In addition, be sure to verbally identify everything in their environment, using baby sign language and words. Sometimes young children pick up on the sign first, then slowly work their way to words.
Also, have them repeat simple 2-3 word phrases by giving them choices. For example, if the child wants a cup of juice but only gestures, have him repeat "juice please" and say, "Do you want the blue cup or red cup?" "OK, you want the red cup." You can also do art activities where you have several steps and have them repeat each step "glue on", "2 white buttons", etc.
I am a speech therapist and I work with preschool children, so feel free to PM me for more ideas and/or websites.
