I recommend cleaning their cage very well before you leave, so they will be safe and then clean it again when you get back. Because the hamsters might miss you when you are gone, maybe leave some toys with your scent on it. Purchasing extra chew toys and stuff to do will prevent them from being bored without you. If the toys are plastic, its probably a good idea to tell the petsitter to only play with the hamsters with supervision with it. Let me know if you have any questions on this reply!
-Aegis