There is a waterproof spray for tents but it doesn't make it like laminating it just makes it waterproof. I don't think it would be good for chair pads, because it would break down if you put any kind of soap on them so you couldn't wash them. I don't think they make anything like you asked about unless it's a sewable plastic and fabric stores sell it for mattress pads. Then there's PUL which has a plastic like inner lining and a cloth outer. They use it for diapers.

Why would you want to coat them anyway--they're washable and dryable and fluff up nicely if you add a tennis ball to the dryer. If you have young kids the. make a slip cover for them or take them off of the kids chairs. I take them off my chairs when my grandsons are eating. With all the wiggling around they do I was afraid of them falling off the chairs.

There is iron on vinyl, but I think you would need to do it before the cushions are made up.

http://www.thermowebonline.com/dyn_prodlist.php?k=82331

I'm not sure if you can do this on cushions already made up, but this tutorial on DIY oil cloth has me psyched - I want to try this myself!

(courtesy of CRAFT magazine) & check out the comments for more info...

Here is a link that might be useful: DIY Oil Cloth