"Tu écris, mon enfant !"
I think it's meant to get used to seeing the words "naturally" versus learning the verb conjugations the old fashioned way through verb charts. I always take this as a supplement versus a replacement to learning French. This is a easy way for "bite-size" learning between various activities.
What if the child doesn't even want to be a writer? This is a very strict parent!
It needs not be imperative. Picture yourself going into your 3 year old kid's room and discovering he as just written his first word.
I actually thought that it was ”you write my child”, as ”you write the book”, making just as much sense as ”green colourless ideas are furiously sleeping”