Bring some mustard seeds to class or home with you one day. Show the children how tiny the seeds actually are and talk about your parable. Have the children glue some mustard seeds to a paper that they can make into a book mark. On the book mark you can write the scripture from Matthew 13:31. Of course you will want to also discuss that important of nurturing the tiny seed so that it grows when you plant it, just like his word grows within our hearts as we learn more about him.