IME, I had cats long before I had rats, and although I have never given them the opportunity to prove otherwise, they are scared of my adult rats! I have cages all over the shop, and they rarely even look in the direction of the rats, I've had rats in carriers on the floor right next to a cat and they don't give them more than a glance.
It's a good idea to keep them in a spare room if you have one, not only because it means that the cats are kept away from them most of the time, but it also means that the rats have their own space, your dcs can let them out in there (assuming it's a safe room, no holes etc), and they won't disturb any sleep etc if the rats have a noisy night at any point.
However having said that my cats don't pay them any attention... never underestimate a predator, I would never allow my rats to run around in the same room as a cat or a dog, even if they've given no sign of being interested by them. You just can't assume that the rat won't do anything like nip the dog/cat in play/fear and antagonise the dog/cat into retaliating. It's not worth that risk especially if you can keep them separate :)
Holiday wise, assuming that you get your rats from a reputable breeder (which, of course, you should!) most breeders will happily acceptyour rats back for holidays, I very often have cages of rats here while their owners are away.
Maintenance wise, they need daily handling and time out of the cage, they need a good quality diet that includes fresh food, they need a good quality cage with good contents (beds and toys, but that doesn't mean expense, rats love cardboard boxes and tea towels strung up like hammocks as much as the expensive shop bought toys etc), they need a good quality bedding that's not pine or cedar based, and they need weekly cleaning. If you get a good quality cage and look after it there's no reason why it wouldn't last for years.
How old are your DCs? You should always assume that within a fairly short space of time, you'll be doing the cleaning and the feeding because they have a party/football/dance classes/tv/homework/anything else they can possibly do that isn't cleaning. Please don't think I'm being rude and that I'm a huge cynic, but 7 years of homing rats tells me that the parents who think their DCs will never lose interest are often very suprised when they are up to their elbows in rat poo a few months down the line.
I tend to recommend boys for young children (10 or under) and girls for older DCs, boys are usually much easier to handle, and much slower, so young kids feel more able to cope with them. Girls stereotypically are less easy to handle, don't like to sit still but that makes them much more entertaining for older children who can cope with the mayhem they can cause, running round generally getting themselves into trouble!
The big expense of rats come with vet bills (if they need them, but lots of rats will at some point in their lives) - rats are prone to things like respiratory infections and mammary lumps. You'll need to be prepared and willing to treat rats when they get ill, and I'd recommend encouraging good habit with your dcs and having a 'fund' that they save for future vet bills/big purchases (even if the money comes from you). I would say an average vet bill for a rat for a resp infection (one off) is approximately £35, and a lump removal can be anything from £60 through to over £100 depending on your vet and the location and size of the lump.
PM me if you need any further advice or a cage recommendation :)