Guinea pigs only get their vitamins from food only on the second cycle. They eat their parents' droppings before they turn a month. So clean their cage each 4/5 days.
Hi and welcome
We strongly recommend to poo patrol the cage daily for those that live on fleece and to clean the cage properly every few days!
Caecotrophs (the redigested poo) are usually picked up from the bum and eaten straight away in order to stay viable. They are never left lying around! You can sometimes see piggies frantically searching for a dropped caecotroph. Caecotrophs are produced at a different time to the waste (or if you wish, second wash) poos and have a slightly different colour, if you ever get to see one!
Caecotrophs are also what you see recovering piggies stick their noses into their healthy companions bums for in order to inoculate and restart their gut fauna after a tummy upset or a course of antibiotics.
Please be aware that we are a strictly non-breeding pet owners forum. Unless a boar is neutered, he has nothing to seek with a pregnant and lactating sow to cause a high risk back-to-back or baby pregnancy. Please also be aware that baby boars gain the ability to make babies from 3-4 weeks onwards and that baby sows have their first season between 4-6 weeks. Baby boars should be therefore removed at 3 weeks of age.
The earliest documented sibling pregnancy is 24 days. I also know of a couple of first-hand cases where impregnation of mum by a large and well-developed son has happened during the second season after birth at just over two weeks after birth.
Babies start eating their own caecotrophs from the word go. They switch to eating mainly solid food between week one and two; the weaning process starts actually soon after. You are right in that they will initially eat some caecotrophs from their elders to help stock their guts with the bacteria that do the digestive work, but that happens actually very early on in the first few days as they can eat solid food right from the start and nibble on it within hours of being born.
Most of the nutrients (including vitamins) come via the milk and not other piggies' caecotrophs until they are able to support themselves. If they really relied so heavily on other piggies' caecotrophs as you say, you would see them incessantly pestering all the other older piggies all the time the same way that recovering piggies can pester their companions. This simply doesn't happen.
Please also note that guinea pigs do not naturally live in strict core mum and dad family groups with just their parents; they are being brought up in a kind of group creche of often, but not always related sows that associate with a boar of their own choice.