Is it safe to leave Guinea pigs out on your lawn to feed?

Hi All, My children have three Guinea pigs which are kept in a large hutch in the garden. Lately, I have been letting them out of the hutch to run around on the lawn (which is enclosed). I feel sorry for them being inside a hutch all the time and they love being out on the garden eating grass. I have a cat and he just lies down by them or walks past. There are other cats in the area tho. What i'd like to know is do you think it's possible for a cat to kill or hurt an adult Guinea pig? The Guinea pigs are quite big and they also have an Impressive set of front teeth! Once they are out on the garden, I sometimes leave them unattended for short periods of time. Anyone had a Guinea pig attacked by a domestic cat before?