YABU I'm afraid.
Six Year Old Child. Not all rescues will see this as an issue, provided the family are clear that responsibility for care of the pet will rest squarely with the adults. Children can help out with the care of animals, but far, far too many pets suffer in a battle of wills between parent and child who was "supposed" to look after the animal and doesn't, has got bored with it, etc. If you are prepared to care for the animals for their potential 8 year lifespan, pay for care during your holidays and cover possible vet bills (one of mine cost nearly a grand over its lifetime,) then there will probably be a rescue that won't mind the fact that you have children.
Outdoors vs Indoors. Guinea pigs DO originate from the Andes, which has a cold but dry climate. Britain has a very cold and damp climate and guinea pigs are prone to fungal skin infections, particularly if kept outdoors. One of mine came from a rescue. He was only 2 weeks when brought in covered in painful, weeping fungal sores. His owners hadn't realised their outdoor-kept pets were so sick until it was almost too late (and they decided they were bored with them, so dumped them at the rescue.)
That's another problem with keeping guineas outdoors. Being at the bottom of the food chain, they are good at "covering up" when they are ill. If you just go out to feed them once or twice a day, you may not spot something is wrong until it's too late. Oh, and the chances of a fox attack indoors is pretty well nil.
Personally, I can't understand why anyone would wish to have a pet outdoors, unless it's physically impossible to keep it indoors (e.g. a horse.) In the US, it's not uncommon for dogs and cats to live outdoors all weathers and people in the UK often think that is cruel, yet have no qualms about having a rabbit or cavy in the same condition. If the animal is outdoors, it's much easier to ignore, easier to think of more fun things to do that don't involve going out in the wet and the rain, easy to "forget" to feed, water, groom and check the animal's health. Mine are in the living room and provide hours of fantastic entertainment!
Size of Cage. Over the years, I think rescues and others with an interest in animal welfare have realised that guineas and rabbits need far more space than thought in the past. When I got my first two in 1998, I kept them in a plastic cage that was way too small. Now, mine are in one that's 1.3 m x 800 cm, but also has an extra floor. The point is, just because the cage you used 5 years ago was "good enough" for the pets you had then doesn't mean that it is "good enough" now (a bit like the cot I slept in probably wouldn't meet standards expected for babies today!)
I've never understood why anyone would use a wooden hutch for an animal that wees inside because before long, it rots through the base and you can't get the stench out, either. A plastic or coroplast floor is much more practical. I find it scandalous that pet shops still sell hutches that are far, far too small.
Sure, you can waltz into a pet shop, hand over a wad and walk out with any animal you like, any cage you want, any food you choose, no need for a background check, etc. The pet shop doesn't care because they just want to make a sale. If you use them for snake food, it's no odds so long as you've handed over the cash. Sadly, alot of pet shops see small animals as fairly "disposable" pets and also sadly, many buyers view them this way, too.
Rescues, on the other hand, know that the animals in their care have already experienced maltreatment and trauma and have a responsibility to do all they can to make sure the animal doesn't end up with a miserable life again. Most rescues in the UK have a "no kill" policy, so even if animals aren't adopted, they'll live out the rest of their lives in the safe and loving care of the shelter or probably a foster family. Sorry, but that is preferable to just handing them out willy nilly to people who stomp their feet and think they know better than the rescuers.
I would suggest giving more time and thought into it before getting guinea pigs. There is plenty of info and lots of helpful people here Guinea Pig Forum.