Usually hamsters trembles. When they do so, that's because they are feeling rather shaky and scared. Thus, by stroking on them more frequently, this will reassure them that you are in safe hands.
Else you can try to put them on a small tray and stroke them, because, sometimes when hamsters smell a foreign smell, for example our palms, they tend to get frightened, thinking we are predators.
Shivering could also means your hamsters has caught a cold. If there is CONSTANT shivering please bring it to the vet -- it is sick.
