Translation:De kinderen lezen een kort boek.
I guess kort is one of those adjectives that doesn't get the added e
No, there is no -e here because boek is a het-word and has an indefinite article in this sentence. See the tips & notes at Adjectives Basic. For example, it would be:
"De kinderen lezen het korte boek."
I see, thanks. Those sneaky het words. I have to start memorizing them :)
Why can't I use lees rater than lezen?
Children = plural, hence you need to use the 3rd person plural lezen instead of the 3rd person singular lees