You could do just their back ends, very gently, suggest kneeling on the floor with bowl with just a couple of inches of luke warm water and small animal shampoo. Rinse with a jug of clean warm water. Near the floor is safer if they wriggle and you let go! Dry them very thoroughly and later comb gently through the hair, I have just trimmed the back ends of my long haired foster piggies, use round ended scissors to do this. This stops them getting so messy and means less chance of tangles and easier grooming. If you don't want to go for a full bath, using a wet facecloth will do a reasonable temporary job, dry them well afterwards.