I recently read an article asking if screen time is causing kids' brains to turn to mush. Fortunately, most kids are not running around like zombies from their brains being turned to mush by technology!
However, if your kids are getting too much screen time, there are effects which may be less obvious. It won't likely show up as academic problems but rather social-emotional problems. This one page flyer has warning signs that your kids are getting too much screen time.