You should review sites like guinealynx.info for information on guinea pigs. You say "pig" but they are herd animals and usually do best in a pair. There are a LOT of rescues and some even adopt out pigs that have already been neutered or spayed to prevent overpopulation. If your DD is generically irresponsible, I would say no. If she really wants the guinea pigs, I would make her earn money for food (pellets, hay and veg) and build her own C&C cage. If she is unwilling to put in the upfront work, she will not want to clean cages at least once a week (or more), keep the water bottles clean, etc. Pigs will smell like warm hay when cared for properly. They will stink if not cared for. A big cage is a must. Do you think she will ignore a pet that is caged? They are very different than dogs or even cats who come to you for attention.
My DD is almost 6 and wants pigs, too, but they will be my pigs if we get more (we've had them off and on for years). I am also well aware of the costs, including exotic vet care. Look at the House Rabbit Society website for vet recommendations - often a rabbit vet will also be good with guinea pigs.
We also have cats and we use an enclosed cage. You can build your own cage with a roof and never ever ever leave the guinea pigs and cats alone outside the cage. Even though our cats are mostly scared of pigs, I would never ever trust them.
Please do NOT get an exercise ball! They are not built to bend that way and it can hurt them.They need floor time in an enclosed place (like your kitchen floor). Please research appropriate toys as many things sold "for guinea pigs" are really for more flexible critters like rats and are NOT appropriate.
ETA: I've had 6 males in the same cage and have successfully introduced adult, unrelated, intact boars. It depends on personalities. Yes, sometimes they don't get along, but that's sows as much as boars. That is also why I suggested a rescue b/c they usually have bonded pairs so you aren't trying to find a roommate. I do agree that shoving the pigs off in a corner (or in a kids' room) is unfair to the pigs and if that's the only option, she needs to wait.