Go to the store and buy some mustard seeds to use in your lesson so that children can see what a mustard seed looks like. You can also bring a book that shows the life cycle of a seed transforming into a tree. For younger kids you can have them glue a seed to the paper and then draw a picture of the tree that it will turn into. For older kids you can have them make a booklet that shows how the seed grows. On each page you can also have them write something they can do to help their love and faith in God grow just like they would nurture the mustard seed.