The first advice I usually give to parents—not that I'm consulted very often—is this: Answer your kid's question, not the question you think they're asking. To illustrate (and I apologise if you've heard this before but it's an old joke): A boy asks his father one day: “Dad, where do I come from?” The father’s been expecting this and sits his son down and patiently explains all about the birds and the bees after which the youngster says, “I see. It's just that Billy next door says he comes from Birmingham and I wondered where I came from.”
A parent has two problems working against him when trying to explain things to a five-year-old: the child has very limited life experience plus a vocabulary probably not exceeding 2200 words. Let's say it's not 'poetry' that the kid wants explained but 'grief'—they overhear someone talking about it—and it's not like you can show them a photo and say, “That's grief.” Amongst those 2200 words I would imagine they would know 'happy' and 'sad' and by five one would imagine they’ve got some idea what death is—maybe they’ve lost a goldfish or a granny—so grief’s “a kind of sadness people feel when someone they love dies.” If the kid wants to know more they'll ask. There's no point in going through the five-to-seven stages of grief. That's not what they asked. And the same goes with my friend's kid. The question was: What’s a poem? not What makes a poem a poem?
How did you learn what poetry was? I'll bet no one sat you down and tried to explain it to you. I discovered a video online a couple of years back—by a guy called George Quasha who specialises in interesting video portraits—and in this video sixty-odd poets struggle to define poetry. It's fascinating viewing (there’s some great stuff here) and it’s also quite laughable at times—Michael Horowitz’s observations are especially hysterical—but it’s also a little sad how tied in knots people get trying to define something that we all immediately recognise when we encounter it even if we too find it difficult to explain to others.
Can you define 'love'? Having experienced a variety of things people call love you recognise it in its many guises. All I would say to a five-year-old who asked me what poetry was would be: “A nursery rhyme is a kind of poem.” If they frowned in that way kids do when it's clear they've not got the point I might add, “You know you’ve lots of different kind of dogs, big ones and small ones, hairy ones and yippy ones? But they're all dogs, right? Well, poems are like that. You can have long ones and short ones, sad ones and funny ones.” If pressed then you can show examples of poems. You don't necessarily have to read them let alone explain them; the visuals will be quite enough. If it looks like a poem then it probably is a poem. It might not be a very good poem but that's a question for another day hopefully many years hence.