Depending on what you find when you remove the seats you could remove the existing padding which becomes compressed over time and use new foam, batting, and fabric. I won't use batting alone as it makes a more comfortable and long lasting seat with foam and batting. Cut the foam to fit only the top of the seat and wrap with batting around. That's what I've done - I don't like to just add to what's there. It's a time consuming job so easier to do it right and it'll last awhile.

I agree completely with luckygal. It's best to start fresh (depending on the condition you find the batting and foam in). I would also suggest using high density foam which can be expensive, but is well worth the price because the foam will be supportive and last many years. Look online for an upholstery supplier - they can cut the right size and height for you. Also, Jo-Anns craft stores has foam, although I don't know about the density of it. The next step is to add a new thin layer of batting over the foam (this rounds out any sharp edges) and finally your new fabric. You can also search for 'how-to' articles for reupholstering chairs - redoing a chair seat is relatively easy. Good luck!