My daughter has a guinea pig (we have a guinea pig). He's wonderful.
Odor: we've had pet rats, mice and rabbits, and the guinea pig poo/urine smells better (still not pleasant when it's over due for a cleaning). At least that's my experience, and that might mean nothing to you if you've never had any of those animals for pets.
A tip: always hold them gently and hold them quietly
while they are little and are getting used to you. We're The Loud Family and it took a little longer for our guy to not be fearful.
I agree, don't bother putting the cage in a child's bedroom. It's just easier to care for it, even if it's primarily one child's responsibility. Besides, the guinea pigs get socialized faster, and they're not so lonely.
Also, I strongly suggest you consider getting two. Guinea pigs are sociable critters that live in groups. While they do just fine on their own, as witnessed in pre-schools around the world, it really is fun to see them living together, and it simply makes them happy. We only have the one (Snickers), and I wish we'd gotten one more with him. My understanding is that it is difficult to impossible to introduce another male to an established male.
I recommend Oxbow brand food. The local mom-and-pop pet supply store carries it here, while we get the giant bale of bedding from Petco. We get this
kind of hay, and it smells divine.