OK, we had two gerbils, they are social animals after all. Good tip - be very sure that wherever you get them from has identified gender properly (was going to say sexed them, but with the tone of this thread I'm not going there). If they have not identified gender properly then one or both may be pregant already!
Proper glass gerbilariums are good, gerbils love to chew, they need to to keep their teeth in order. Plastic cages become something else to chew, so the glass ones will resist that but ensure you have plenty of chewing toys for them. We used to re-cycle all of our toilet rolls through the gerbil cage and get suitable wood toys for them from the LPS.
Ours were pushing two by the time they started to pass away, so they can live reasonable durations, but as Pete says it is important to ensure the kids are aware that a gerbil lifespan is shorter than a human or other larger mammal lifespan. They will also die at inopportune times, and kids grief process can mean it isn't as easy as putting them in the bin! First death in our gerbils resulted in plans for a funeral in the garden (not quite, nut I am sure you get the drift!!). Second one the child dealt with much better, but then I'd been seriosuly ill, so I think he'd gained insight through his experiences and coped with it better.
They should be handled carefully, they will shed tails if they are mishandled by their tail, although this does not cause them any long term problems. They do need handling though to socialise them, otherwise kids get quite bored if they can't get them out and hold & stroke them. they can have a tendancy to experience growths/tumours, ours that lived to 2 1/2 had a growth on its abdomen but this never appeared to cause it a problem and when we discussed it when we took the dog to the vet the vet sort of shrugged and went "it happens, we can destroy it if you want?"
Agree that the ball type run around devices are good, but consider that gerbils, like most rodents, can't control bowels and have a negligible bladder, they will pee & poo constantly. Clean them out regularly and use a pet-safe hygeine spray to clean their tank etc. Food wise the muesli type stuff is good, but then the muppets will often only eat certain parts of the muesli. Other treats can be got, but ours never seemed to touch those.