DH thinks DS is too young for pets. Granted we have a rabbit and a cat but the rabbit is alas very unpredictable and aggressive so I won't allow DS near it. Cat is more placid but generally lives outside and in conservatory and sleeps all the time.
DS loves to look after things and when we went to the farm last week he say stroking a guinea pig for over half an hour and asking about what it ate and where it lived etc. I know it was partly novelty but he genuinely got a lot of pleasure from the little creature. When it licked DS's hand DS said 'Ahhh! He likes me! He's giving me kisses! I'll look after you little piggy.'
I think DS would be quite into helping with feeding and grooming even though he is only little. Obviously I would be ultimately responsible and would clean them out and supervise DS with them to make sure he doesn't hug them too hard. DH says DS is too little to properly appreciate a pet. I think learning to care for something and look after it can only be a good thing.
Wibu to get two female piggies for DS? Hamsters are too small and generally piggies have a gentle temperament from past experience.