I have sunbeam ones from costco, but I don't find them that warm, I'm not sure how great they would be outside. I also sometimes worry a bit about my kids using that or the heating pad and leaving it, forgetting that it's on.
I think you would be better off with some really thick blankets, the material around the heating wires is thinner and heat radiates up.
You could also try heated seat cushions with regular blankets. I think that would work better outside.