Children can learn the conventions of poetry very early if exposed to poems or songs. Exposure is the single most important tool of teaching poetry skills. You do not need to tell children to write poems, just read to them and soon they will be reciting their own creations to you. Be ready to write them down. If you haven't read any poetry by young children, start looking or listening.
Children also break conventions very early.
Be ready for occasional brilliant surprises. Here is another by Alex at age 5. Here is a seasonal poem by Sam Logan. Often children's words are worthy of recording as poetry, even when they are not thinking of poetry. Found poetry is a form which just records the naturally occuring poems in advertising, news articles and such.