I have no affiliation with this site, but I went searching on Google just to see what I could find and ran across a company that at least had a good listing of Fire Alarm covers:
http://www.wrhw.com/catalog/customer/home.php?cat=320
I KNOW that I have seen the versions that you have to break the glass with the little metal stick before you can release the alarm handle before. I lived on college campuses for about 7 years (4 years undergrad, then 3 years while the wife worked in college housing) in resident halls and I remember at least one of the halls we lived in had this type of alarm to at least deter all but those who are determined to do something vandalistic.
I would suggest one of the covers that has a glass front with the little metal handle to break the glass. I think that would be something not most children would attempt just out of curiosity.