Hello Spike
9yo is a good age to get guineas , I was 9yo when I fell in love with the school piggie (and kept pigs till I was 22yo) then my own DD was 9yo when she wanted a pet and I steered her towards piggies . We had eight of them over the last eight years .
You will get into a cycle , one dies , you get another (and in our case ended up at one point with 5 )
You of course will be responsible and much as your DD says she will do all the cleaning and feeding -she won't . My DD was brilliant we shared PigCare but of course there were times she was ill, away , busy with homework or simply CBA .
The "Mum can you do the pigs" was always met by "Yes , half of them are mine but you do breakfast"
As the Adult YOU are responsible and YOU pick up the slack for the lifespan of these gorgeous little animals .
Get your DD to cuddle some pigs and handle hay . Allergies are very common. My DD was fine til we got GP3 who was a coarse coat and GP6 who was very allergenic (Rex and Teddy) . She wouldn't let me have GP6 and got red rashes when she cuddled him.
So for breeds - I like a nice lardy Smooth . Bog standard , no frills .
Abby are lovely (rossettes) but stubby little muzzles and more risk of cataracts .
Long haired are hard work , more grooming .
Breeder or Rescue ?
Rescue is more ethical. Our pigs were Rescues but all adults except GP3 (he was taken into Rescue from a Breeder at just 6 weeks old)
While your're planning:
Bars or Sows . If you get an adult rescue boar they often castrate them and bond them. A little herd is a delight .
Boars are soppy but you need to provide for them. Space (lots) understanding their relationships .
Sows are lovely , IME untidier 
Where will they live ? You need a big space . Ignore those shit little cages that are nothing more than a litter tray .
If you want to keep them outside they'll need protected - heat, cold, damp, draughts, predators
Your DD will need to go out to clean the bedding , feed , check , cuddle .
Or indoors you need to keep on top of things , they are rodents ! Hat smells far more than they do but if you don't keep them clean the bedding can whiff
Hay will be the bane of your life .
Guinea-pigs are he gentlest , chattiest , greediest , most vunerable little animals . They don't **
kick
bite
jump
climb
** our GP3 kicked if you touched his side , nothing painful, he just reacted . He 'airsnapped' if one of his wives ignored the warning
GP5 was a gnawer , but not a bite
GP3 also jumped out his travel box which was mt knee height . He didn't wait to be lifted , as soon as he was on the floor and the lid lifted , he flew 
They also get ill and die very quickly and they hide it . You can find a local vet that specialises in small exotics .
They don't need vaccines and most potter through life happily , but ilnesses are tricky , they don;t let on they're ill.
I'm going to find something from Guinea-Lynx and post it .
It's poignant