Are their paws bigger than an ink stamp pad? I'd press stamp pad against paw, then fabric (attached to something firm) just for insurance, I'd outline the print with a sturdy fabric marker or embroidery. I'd really stick to anything going on the paws as non-toxic and washable.
I've used acrylic fabric paint to do handprints and it worked fine, but it would sink into calloused puppy toes. A lot depends on how well your dogs like baths and "stuff" on their paws.