I would have to agree with everyone else it is more likely that the pups mother ate the pups. She would never let the cats near her pups.
To be honest and im not being nasty but the cats should never have been allowed anywhere near the bitch and her pups, the bitch could have killed them, I never leave my pups alone at such a tender age... anything can and does very often happen just because mum can be so easily spooked and squash or eat them. Such a tragic story !!!
RIP little ones !!!!!