Thank you for considering Kiddiz. Here are your answers.
1. Go to dashboard -> appearance -> customize to change the site identity and background color/images.
2. You can’t remove header image from pages if you remove the default header image will appear. It is done to justify the layout of the theme.
3. Please set featured image for each page to have different header images for each page.
4. Please use classic block to get formatting options for paragraphs.