You don't need to be an artist to sketch out a couple of ideas for how your character should look. In fantasy books you get to choose what race they are and my favourite part, which weapons they will carry. There are so many options like the colour of skin, hair, clothes, height etc.
Secondly you think about their personality. What motivates them to act a certain way. Has something notable happened in their past? What was their upbringing like? What are they trying to acheive in your story?
The example I used in my talk was on Huntress Sienna, a halfling who lives alone in the Wild Woods of Reloria and has learned to do everything for herself. Because she has had no maternal guidance, she dresses like a boy and is dirty with tangled hair. When she meets other halflings, her who world changes.
I showed the students a great book, entitled 'Child Writes' by Emma Mactaggart which takes children step-by-step through the process of writing a book.
Happy writing kids and best of luck with your characters, cheers, Kasper.