Talk about it - talk about how it's easy to use this app because the designers made it easy to use, or it's hard to use this app because the designers made it hard to use. When reading books talk about the images and the fonts. (For example, Sam and Dave Dig A Hole has lovely drawings, large print, and a clean simple font with high contrast. Farther has a weird font in a low contrast.)
Involve them in decisions about decoration and room layout. Introduce simple design and art concepts early, and help them spot patterns. "Colour wheels" are simple enough, so you could buy a bunch of paint and do some potato printing with complementary colours, and clashing colours.
https://www.amazon.co.uk/Sam-Dave-Dig-Hole-Barnett/dp/140636...
https://www.amazon.co.uk/FArTHER-Grahame-Baker-Smith-x/dp/18...
PS: If you're involved in children's books, and you want to use a weird font: just fucking don't.
Picture books for young children- We really enjoyed I Want My Hat Back, This Is Not My Hat, and "Sam and Dave Dig a Hole" written by Jon Klassen and illustrated by Mac Bennet.
https://www.amazon.co.uk/White-Read-Aloud-Award-Picture-Book...
https://www.amazon.co.uk/This-Not-Hat-Jon-Klassen/dp/1406353...
https://www.amazon.co.uk/Sam-Dave-Dig-Hole-Barnett/dp/140636...
Books (with some illustration) for older readers- We're enjoying The Saga of Erik the Viking by Terry Jones and illustrated by Michael Foreman.
https://www.amazon.co.uk/Saga-Erik-Viking-Puffin-Books/dp/01...