Ooh, that’s really tricky, isn’t it! Because usually you’d have a finished book to show them, you wouldn’t be asking for feedback on rough sketches (plus, I know my 6yo wouldn’t have much to say besides, “wow, that’s really good, mummy!”).
The only thing I can think of is printing off finished coloured versions of your characters and reading your story while using the characters like puppets. At least then you can engage them and get feedback on your story and characters.