I have used a bait box before now without mishap. Next door has a very run down shed with rubbish piled round it which is rat paradise, so every now and then we get unwelcome visitors. Rat poison works quite slowly so they tend to die underground during the day.
I take the food in every night and that is really one of the best ways to deter them. No reason to come if there is nothing to eat. If you are seeing them during the day, they are getting bold and you might need the pest man back in again.
Would a temporary run area work for you? The hens would only need to be in there for a week or so and then you could be sure they were nowhere near dead rats. A pop-up or folding rabbit run can also be used, just make sure you shade at least half of it