Building a good vocabulary is very important. Reading is clearly very influential in building up a vast array of words to use. Here is a tip to help your child
1) Get your child to create a vocabulary book and label the pages A to Z.
2) Ask your child to look up at least 5 new words each week and write them and their meanings into the book in the relevant area.
3) Then top it all of by encouraging them to use the vocabulary book when doing any sort of regular writing. Forming a daily diary works well with some students. Words put into context will have a higher chance of being remembered.
I do hope that helps. Watch out for TIP 2