Just my 0.02...I personally would not do a ferret if you feel you can't manage a dog or a cat. Ferrets are extremely active and inquisitive - they won't be happy just being in a cage all the time. They need lots of exercise and attention and they get into everything. Plus, they smell. All ferrets sold as pets are spayed/neutered and have had their main scent glands removed, so they are not as bad as they could be, but they are still pretty stinky. And they require more medical care than you might think, especially as they get older - they are prone to adrenal gland problems and a whole host of other issues, including cancer. I am not trying to knock ferrets, I know many people who have them and love them and they can be a lot of fun and very amusing and friendly, but personally they are just not my cup of tea. Like any pet, there are things to know about them and take into consideration before getting one.
Rats are highly intelligent and can make good pets if they are handled from the time they are babies and used to human contact. I also like guinea pigs, since they are also relatively tame and easy to care for if they are used to people. Any kind of decent sized cage should do. However, the kids should be supervised at all times if they are handling any animal. Also, many rodents (as well as reptiles) can carry Salmonella, so that's another thing to be aware of - make sure the kids wash their hands really well after handling them. A rat's average life span is around 3 years, while a guinea pig is usually 5 to 6, if they are cared for properly.