Here is one article of many on multi-tasking.  this one is pretty good becasue it offers actual advice you could share with your student's parents.  Just "google" 'dangers of multitasking' to find lots of adult-oriented links too.

http://www.scholastic.com/resources/article/the-perils-of-multitasking

This one is specifically addressed to educators.  I think it is worth reading too. 
http://www.commonsensemedia.org/educators/blog/expert-article-kids-and-multitasking

Even if kids aren't overwhelmed by multi-tasking, it doesn't mean that it is good for them.