I kept sea monkeys for nearly a year I think. They are really Brine Shrimp, which hatch from freeze dried eggs out of a packet.I wouldn't worry too much if they all disappear, as your neighbour's daughter might already know from the manual, if they all die then you simply let the water evaporate and fill it up again with more purifed water and another sachet of whatever it is and they will magically come back to life in a few days. From what I can remember. Have a read of the instructions, it should explain the whole process more clearly and scientifically than I can or go to this site:
http://www.staff.ncl.ac.uk/andrew.harbottle/seamonk.htm there is an official site but I can't remember the address, it should say in the manual somewhere. hope this helps!