We had 2 cats when we first got chickens - one was huge - and all muscle. Neither cat ever went after any of my chickens (and I want to say I started free ranging at the about the age yours are now) and we get baby chicks every year. I do not think it is necessary to spray the cat at all. I would watch what happens and if you are out with them and the cat looks at them funny (which I kinda doubt) just make a loud hissing sound while moving toward the cat and I am sure the cat will get your message. If you are still timid about it (better yet)- hold the cat in your lap and let the chickens out so you can see how the cat responds directly (or let the neighbors hold it - kudos to you for being good neighbors!) and see if any intervention will be necessary.