Sometimes it is best to have each keep his sentences secret until you all read them aloud. Sometimes you may want to give each line out loud right at first, expecting the first ones to influence what others may say, and thus build a more harmonious poem. Try both ways. After each family member has contributed, you may want to rearrange the sentences to make more sense or create more unity in the poem. Any subject you are all interested in would be a good topic to start with.
Little children like to express themselves.
Let them say whatever comes into their minds. Some of their responses can be very revealing and surprisingly wise. Make up many more than the seven examples given. It will be fun for you too. This form of Japanese expression and appreciation for nature can be a really creative experience for the adults in your family.