this may sound really silly but do i need to wash the grass? we have no treatments on the lawn but do get the odd cat... once the run is down i will know that area has not hopefully been peed on...Hand pick some at first and feed it to them over a few days so their tummy gets used to it. Then they should be fine to go on the lawn as long as it's shady and dry
I'd build up the amount of time you give it them. Eg 30 mins one day, then leave them longer on it the next day
They will love it!
It's true though isn't it when you pick grass you want to wash it but I doubt anyone hoses their lawn and waits for it to dry before they put their pigs out. It just wouldn't be practical.