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.
Show them how design is responsible for every day things that they're already using. Take them to an industrial design museum, perhaps a conference that focuses on 3D printing so they can see with their own eyes how everyday things are created from scratch. Next time you drive past a stop sign, ask them why they think it's red - that alone opens up doors to discussions about everything from psychology to evolution.