S.B.
This has happened to us, more than once :(. I simply put the carseat on the back porch and used a cup to control where I poured the water, but I still doused it down pretty good. Then I set it out in the sun. Fresh air and sunlight seem to help air out many things. After one afternoon it was better. To help with that issue while potty training, I cut down some old waterproof lap pads to fit over the bottom of the seat. Usually, I still had to wash the cover, occasionally I got lucky. Either way, it controlled the mess better until my kids were completely potty trained.