I kept rats for years and they are lovely. They are social creatures, unlike hamsters which are solitary, and seek out and enjoy human company.
Buy a cage that is much, much bigger than you think you will need as it will probably be about the right size.
I used wood based cat litter to line the cage and yes, you do need to change it regularly. In my experience some ratties smell more than others - in fact it was my last three girls who were the whiffiest of all, but you can keep on top of it by cleaning out the cage vv regularly.
My cautions would be that, as for all caged pets owned by children, you will be doing the lion's share of their care and upkeep and this will include cleaning out their cage thoroughly at least once a week and lugging around bags of food and litter.
Any caged pet also produces a lot of dust and extra general cleaning, and you will need somewhere to store their supplies. They do not live for very long, which is hard enough to deal with as an adult, let alone as a child.
They will also usually need a trip to the vet's when you first get them to clear a (non infectious to humans) mite infestation that they all seem to get. One injection will clear it.
Ensure also that you buy your rats from a reputable breeder or decent pet shop. I have come across pet shop owners that do not separate male and female babies to ensure a constant supply of livestock which can lead to severe health problems due to inbreeding, and temperament issues due to lack of any handling.
I have two cats now - I loved my rats but really, cats are so much less bother