Hi fleas there are loads of guinea related threads for you to have a peruse of here
To answer your queries here though
(not in order, some area easier to answer)....
4: they don't need vaccines and they don't need neutered unless you want a non-breeding sow/boar pair.
You'd need to protect against mites , treat if they crop up (I use a spot treatment on their necks)
5 : cost: a bag of pellets £9 /4kg lasts my 2 boars about 4-6 weeks but they waste alot. (With boars it's better to have extra food so they don't fight)
Hay - check out local petstores and hay suppliers (I got some naice hay £20 for 16 kg)
Veg: in theory your pigs will eat pretty much all the veg you have (except potatoes, iceberg lettuce)
But if you are anything like me the rest of the guinea keepers here, you'll be buying parsley, leafy celery, corn-on-the-cob and thinking "Oh, they like romaine and spinach. I'll buy that"
3: You can keep them outside if they are very well insulated but they don't handle cold and damp well so you'll probably want them inside for winter (shed or garage that the car doesn't go in)
2: yes, you need more than one. Two girls or two bonded boys. Bigger groups can work but not more than 2 boars and not 2 boars with sows
1: age/size : baby pigs are cute as a button but very skittery
We had two adult boars (Rescue) then last July one died (the quieter one). The big boar brother was bereft alone, my daughter didn't want to neuter him (to sow match) so we got a little boar piglet. He's now about 4 months old, very nearly the size of his "Step Dad" and just settling down.
Though we don't know if he was handled much before he came to us.
Your DDs are young and though GPs are very friendly but they are big and though they rarely bite (there's always a reason if they do) they have a sharp set of teeth.
My DD is 11. If she gets a bite it's because either her hands smell of food and they've mistaken her hands for food or if they aren't lifted properly. But even then they'd rather shreik than bite.
Can you take your DDs to handle some guineas , make sure they're not allergic to fur or hay.
Good Luck