Why? there are some excellent non-fiction for 7 year olds. In UK 7 year olds are learning science and math in school.
eg The Way Things Work https://www.amazon.co.uk/dp/0241227933/
The Awesome Body Book https://www.amazon.co.uk/Awesome-Body-Book-Adam-Frost/dp/140...
Just a Second https://www.amazon.co.uk/Just-Second-Steve-Jenkins/dp/132874...
Some children are naturally curious and can't get enough of this stuff.
STEM is a non-word used to discount art and literature and dance and history and everything expressive. It reflects the idea that human existence is about getting ahead versus falling behind relative to the conveyor belt created by the Sputnik Crisis. Absent are notions of blossoming and flowering and joy. STEM is white papers by think tanks and research grants and educational politics. STEM is an agenda.
A book about trucks, ships, and planes is a book about trucks, ships and planes.
I understand the appeal of introducing your child to your passion, particularly if it's an employable skill, but IMO at 7 years old... just buy them whatever books or encyclopedias for children they find interesting. It's not going to hurt them! Though definitely share technology with them.
I'm curious why you think I'm not doing this? Why have you assumed I'm imposing what I want my child to read, rather than listening to what he wants?
Neuro-atypicallity exists, and some children do not enjoy fiction.
So a book like Budgie the Little Helicopter is a book about a helicopter, but I know my child will prefer a non-fiction reference book about helicopters. And this non-fiction reference book is a STEM book.