Show them how to proofread if they don't know how. Do not badger them if they miss many mistakes when they proofread. At first they may miss a lot of their copying errors when they proofread, but they will improve with time. Once your children are proofreading very well, then you may wish to give them longer selections to copy, perhaps 10 to 12 lines. This time as you find selections for them to copy, look for certain elements of punctuation that you wish to teach them.
Quotation marks are quite easy to learn first.
When they learn quotation marks, they will also learn how to use commas along with the quotation marks. Remember to have them proofread their work once they are finished. Move along in this manner as your children progress. Their third year is also a good time to introduce them to action verbs and concrete nouns. Eventually copy-work can develop into dictation.