Guinea-Pigs are lovely , my DD and I have 5 at the moment 
First of all, your DS is 4, which means you will do 99.9% of the work. Your 4yo can feed and help to clean but it falls on your shoulders.
They need :
Company (other pigs)
Space (for 2 piggies at least 4'x2' but bigger is better) Those hutches you see with the open area/sleeping area? See how small the enclosed bit is. The wire meshed bit is open to the elements, guineas like to snuggle
They need protected: against wind,cold,DAMP especially, predators (cats,foxes,wild rats) stress , themselves (they are phenomenally dense. I say this with love, I have kept pigs for years and this to me is their main attratction).
They don't tend to climb or jump (mine don't cope with ramps except to hop up into their hutch/run) and 99% don't bite. I have a biter but mine are all Rescues, she's allowed to.
I wouldn't leave mine outside all year TBH, they would just huddle in the haybox and that is a miserable existance.
I have a wooden playhouse for mine they have hayboxes (cleaned every day) and floorspace.
We have big outdoor runs for nice weather.
They can get very ill very quickly ( Well-To-Dead) so you need to be aware of ailments, what they can or cannot eat and have a Guinea Savvy vets details . You might never need a vet but its good to know where they are.
Once you have two, you're in the Spiral of never ending piggies. DD started with two boars GP1 and GP2, now a few years on we have GP4 GP5 GP6 (our neutered boy) GP7 and GP8 (our newest girls)
Boars are lovely and snuggly but like all male rodents they have their (not unpleasant) boar smell. They need a load of space.
Can you ask at the nursery about your DS maybe helping with the piggies at a weekend? Do they have someone taking them home at all?
My DD used to help with the School rabbits in Yr6 but she much preferred her guineas. 
And you need to make sure your DS isn't allergic to the hay or the fur.
My DD is fine with smooth coats but very allergic to the coarse ones (when she cuddles the boar GP6 she gets a rash within minutes) I offer to take him (I'd never rehome my hogs) but she puts up with the rashes. Maybe because she doesn't want my bitey pig.
Their mess gets everywhere, mine come in for winter nights and I find their debris all over .
Much as I love my piggies ,maybe think about a friendly hamster from a reutable source, one that is handled and tame. Downside is the nocturnal traits.