What a sharp 5-year-old! And a tricky one to answer. I've written about language for young people, but never as young as that. A Little Book of Language was primarily aimed at young teenagers - a memorable experience for me because, to check I'd got the level right, I had it read by a 12-year-old. I'd rather have a book critically reviewed by Chomsky! She pulled no punches.
In 2012 the NSPCC published a lovely little book called Big Questions from Little People. It took 100 questions asked by children and got experts to answer them. A few were linguistic:
Why can't animals talk like us? (Noam Chomsky)
Who wrote the first book ever? (Martin Lyons)
How did we first learn to write? (John Man)
Why do we have an alphabet? (John Man)
Who named all the cities? (Mark Forsyth)
Why do we speak English? (me)
It was so successful it published a sequel in 2013 called Does My Goldfish Know Who I Am? This time the language questions were:
Do spiders speak? (George McGavin)
If you shouted in space, would you hear anything? (Ben Miller)
Do animals like cows and sheep have accents? (John Wells)
How do we learn to speak? (Gary Marcus)
Why do we have books? (Maria Popova)
Do babies think in words or their own language? (Charles Fernyhough)
If oranges are called oranges, why aren't bananas called yellows? (Philip Gooden)
Is silence a sound? (Quentin Cooper)
Why do cats 'miaow', cows 'moo' and sheep 'baa'? (David Bellos)
How many languages are there in the world? (me)
Some of these questions were asked by children as young as four. Finding a way to answer them that accommodates sucessfully to the age is really hard.
For 5s, the ideal approach, to my mind, is to create stories with appealing characters, plots, and illustrations, and I've not come across many cases where writers have tried to introduce linguistic metalanguage (basic notions, such as 'words') in a story-telling way. Usually, such writing is for older children (aka adults), such as James Thurber's The Wonderful O. I devised an entire programme on this basis once, called DIAL ('Developing Ideas About Language', aimed at primary kids, and entirely story-driven; but (this was the 1980s) the publisher who commissioned the idea never went ahead with it. I've since been keeping an eye open for similar material. A lovely example is Cynthia Rylant's The Old Woman Who Named Things (1996). Maybe readers of this post will be able to provide some further instances: the crucial point is that the story must focus on at least one metalinguistic term.