What is the best way to keep slugs out of my outdoor cat food bowl?

I have a cat that has lived indoors for most of her life, she is 11. Since we brought our daughter home nearly a year ago the cat has peed in the house. We have tried EVERYTHING short of getting rid of the kid to make the cat happy. We have just had to spend several hundred dollars replacing part of our floor due to cat urine. Sadly, I have put the cat outside as a last resort. She has food, water, shelter, etc but I cannot keep the slugs from getting all over her stuff. Any suggestions? Are slugs beneficial? If I should get rid of the slugs, how can I do so without hurting my cat?