Good communication is two-way and requires good listening skills. To help a child, you will need to demonstrate good listening skills yourself. Make sure that you have time for this in your day. You may need to explain words that a child still does not know.
A child's vocabulary will be growing. Help them to understand new words they learn. And make sure they are not afraid to ask if they don't understand a word. If you don't know the exact meaning of a word - look it up in a dictionary.
There are lots of online dictionaries, like http://dictionary.com/ that you can use.
Just by having good conversations with children, you are supporting their language. So talk to them. Ask them how their day at school was, and how their friends are. Hopefully they don't need too much encouragement to talk. Try to encourage conversations rather than just talking.