Hi Bugsy - I'll try and give you some answers 
We are thinking of getting guinea pigs for our girls, 14yr and 10yr First off (lets just get this on the table straight off) You aren't gettng them for your DDs . They will be Legally, Morally and Practically Yours . If you don't want to be responsible for their care , then don't get them. Sounds harsh ? Yes. My DD got her first guineas at 9yo, I kept guineas from 9-22yo ( so she was always going to have guinea-pigs rather than hamsters or rabbits) . My DD was Brilliant but there were times she was away, ill, busy with homework or knackered. And it was "Mum can you feed/clean the pigs"?
Half the pigs were mine so yes, we shared but sometimes, I did a lot more .
. Just needed some general advice! Is it better to have males or females? Personal choice . Boars are soppier but you need to give them what they need . A two male pairing is not what they would choose to live in ( in their natural state you'd have a boar and a group of sows). If you give them space , don't let any sows near a boar pair and watch for any signs of teenage temper , they are gorgeous.
Our pigs (we had four boars and four sows over the years.All rescues. The sows were the feisty ones
)
Obviously we would need a suitable cage (how big?) and a run Guidelines say 4'x2' minimum for 2 adults but that is a pitiful existence . I gave my guineas a wooden playhouse (DH customised it) the floorspace was 6'6" x6'6" . They had on -the-floor hutches with cut out doors so they could sleep in the hutch and walk about , boxes, food bowls , waterbottles .
Indoors we had C&C (grids) and a 4'2' (which they pretended to be judgey at ) only to sleep in at winter nights .
When our last 3 were oldies we kept them inside all the time in their last winter .They had the small bedroom which has no door . Piggies are noisy ar 3am 
Ours had rabbit runs but we put them in. They cannot tolerate damp, cold,heat,wind so I wouldn't trust them to choose . They are not the brightest animals. They don't need 24/7 access to grass.
. Would they need to be shut in on an evening? Yes 100% . They are vunerable creatures . Cold, damp, predators . (Foxes, badgers, )
What do they need with regards to bedding? Would it be newspaper and hay? There's no holy grail. Mine liked hay . I used (outside) thick rubber mats + cardboard . Then lots of newspaper on the floor .
In their hutches cardboard, puppy pads , newspaper + hay .
Indoors (especially on the C&C grids) thick card board , puppy pads , newspaper , hay .
We cleaned the bedding area every day . Floor space maybe weekly .
In Summer you need to stop them over heating . We switched to shredded card (either equine bedding or shredded looroll tubes)
Covered up the rubbermats and puppy pads , luckily mine weren't chewers .
Make sure there's no staples or selloptape .
Can they stay outside all year round? It's do-able but not much fun for them. They are sociable little animals . In winter ours went out to their Pighouse by day , in at night . You need to make sure the temperature is constant . Hot-to-cold or cold-to-hot can kill them.
And your DD won't go ou in Winter to clean or tend to them, no matter what they promise
We do have a small shed that we could move them to in very inclement weather but would only be able to have a small cage in there How small a cage?
.
Final question, we have two cats, one is a bit of a hunter (mice and small birds!), does this rule us out as good guinea pig owners?
After 8 years of guinea-pigs ( we had eight of them over this time , at one point we had 5) with all the hay getting everywhere , the buying and storing of bedding (huge bags of hay) dirty bedding bins, buying the veg. Cuddling an assortment of of piggies .
When they go on holiday you need to arrange something.
When they are ill they get ill very quickly . They hide it well, sometime too late .
My DH wanted cats . I wouldn't while we had the piggies . So after our last old girl died , we looked for cats .
Our cats are fairly predatory so I wouldn't have wanted them near my guineas .
Guinea-pigs are prey animals and spend their whole lives assuming they will be eaten.
I know lots of people keep guineas and dogs or cats .
But I wouldn't personally . I don't think it would be fair on the cats "Mum there's something there can I kill it? Can I?"
That was an essay 
If you go ahead , make sure no-one in the house is allergic to their hay or fur . My DD was ok with 1&2 . Allergic to 3 . Very allergic to 6 .
She wouldn't give GP6 to me , so she had red rashes whenever she touched him.
Thanks