Pick up the cat first, then select the flea bath option (I think it helps to keep the bath tub in the yard).
Also, you can cheat to get the pet hygienator.
1. Open your cheat window by typing CTRL+Shift+C then type in "testingcheatsenabled true" (without the quotes).
2. Then open your cheat window again and type in "buydebug on" (again without the quotes).
3. Go to buy mode and click on the question mark. Scroll through and you should see the Pet Hygienator, which your pet can enter on his/her own. It will get rid of fleas and give your pets a positive moodlet whenever they use it.aaa