I Love my piggies, I've kept piggies for years, since I was 9yo till 22ish then my DD got us on the Piggy Wagon again when she was 9yo , we have GP4,GP6,GP7,GP8 (GP5 went To The Bridge yesterday, Bless her) That's a downside, they leave a huge hole in your heart when they go .
Mmmm dogs, cats and piggies? They are prey animals so they might be ok, but deep down they'll always feel like they're Lunch which is a shame because once they get brave and tame they're fantastic little rodents.
Cats are natural predators (my DBro cat visits but we keep the pigs in their shed, not fair on either to put them in a risky situation)
They are messy- the hay they cast about without a second thought (my DD was looking after ours last week, she's now 14.6yo and she did an excellant job , but the hay is EVERYWHERE
)
Lots of DC are allergic (DD is allergic to Rex and Teddy fur, our GP3 was Rex, GP6 is Teddy)
And the hay - hugely allergenic.
They need protected from cold,damp,draughts,heat,predators. So unlike rabbits, its hard (but not impossible) to keep them outdoors all year.
Rabbits aren't considered by many to be a good childs pet (I've never kept rabbits) but piggies are a considerable amount of work too.
You'll be doing 90% no matter what your DC tells you.
You'll need to store hay , (shed or garage?) plan where they'll live, an outdoor run+safe shelter, safe hutch or cage.
You'll end up in an onward spiral when one goes, you get another and on and on.
They don't always get on. My new girls don't like my original ones.
My boar was in Rescue and had to split from his brother (fighting) so he was neutered to rehome.
Has your DD expressed an interest in piggies or would she rather have rabbits?
The things I love about guineas is they stay on the ground, don't run(much) climb, kick (except GP3) , bite (except GP5) but they are not the sharpest knives in the drawer. Some people find them boring 
I quite like the idea of free ranging rabbits in the garden at dusk on a summers evening.....................(not enough to make me get some)
Most guineas end up in Rescue or Gumtree because of Kids Lost Interest/ Kids Got Allergies or " Not getting on with other animals/dog won't leave them alone"