When I had a swimming pool, I'd drop the cushions into the pool for a day and the chlorine in the pool water would clean them up and kill any bacteria responsible for mold, mildew, etc.
Of course, you had to allow a couple of days for them to dry.
If you don't haver a pool, sponge them down with a bleach water solution and let it dry naturally. Check the bleach bottle for proportions for tough cleaning jobs.