I think they are just normal humans who were raised from infancy to serve their vampire masters. I don't think there is any mind control other than a lifetime of indoctrination. In return they likely get a good (well, probably still below average for any other human civ) diet and sanitation, so that they are healthier and tastier than the slaves that make of most of Calabim society. They are likely taught that it is an honor to be fed upon, so that unlike other victims of the vampiric ritual they will not resist. Willingly giving their souls up may make the ritual faster or more efficient, although most vampires are probably sadistic enough to prefer torturing their victims into submission first when they have enough time. I would not be surprised if Bloodpets were taught the first steps of the ritual to help prepare other victims for their masters, and even perform these steps on themselves so as to save their masters time.
It is also from the ranks of the most loyal Bloodpets that the Moroi are chosen, and given a chance to earn their initiation into the vampire aristocracy.
