They can be entertaining but they don't live a really long time. Birds are cold-blooded so they need to have plenty of seed for constant nourishment to keep their body heat up. Cleaning the cage isn't fun but it's no fun for any other animal either. Be aware that you cannot use cookware with a non-stick coating at high heat - like cookie sheets or cooking with a non-stick fry pan. The non-stick coating gets into the air and is toxic to birds, who are sensitive to various gasses. That's why you always heard about canaries in the mines in prior generations - when the bird died, the miners knew gasses were building up. (Kind of makes you wonder how good those non-stick coatings are for US, too, huh???)
If you do get a bird, I would consult with a really good pet store - preferably one that is independently owned rather than one of the big chains. At the chains, you don't know if the person working there is just a clerk or if they really know anything about birds. At a smaller store, be sure you talk to the owner, not just a clerk. Also consult with any vets or veterinary techs that you know, or a veterinary college - find out what they see a lot of or what they recommend. You'll need a vet anyway, most likely, so it's a good time to find one in advance and get some advice about what treatments are necessary to keep a bird healthy, what breed is recommended, what breed is inbred or overpriced, and so on, where to keep them in a room (distance from radiators and windows, for example, and so on. Also be sure you have a cage your girls cannot open. Trying to catch a parakeet flying around the kitchen is no fun!
Hamsters and gerbils ARE rodents, you are right. They can be fun to watch though if you have those tubes they run through. They can be hard for a young child to handle without hurting them. Guinea pigs are bigger and possibly more interesting. FYI Do not get a ferret - they are adorable but they are up all night and do nothing all day.
Remember that, no matter what type of pet you get (fish, bird, mammal), there is care and upkeep involved, and no matter what your children promise you, they will NOT do the work after the first 15 minutes, and it will be entirely your responsibility.
Good luck with whatever you choose. Another fun thing might be to get a membership to a museum that has animal exhibits and special shows featuring a particular animal. See if they are really interested or just talking about it now.