Since guinea pigs are mammals, they’ll drink their mother's’ milk for the first few days of their life before gradually moving on to solids. If you’re unlucky enough to have a very young baby guinea pig rejected by its mother then you’ll need to hand-feed it.
Hand-feeding should be done with a teaspoon rather than with a syringe, as baby guinea pigs can easily breathe this in by accident. You can use full-fat goat’s milk, or make a milk mixture that is half water half evaporated milk. If you soak brown bread in the milk, then offer it to the baby guinea pigs on a teaspoon, they’ll be able to suck the milk out.
Baby guinea pigs may need you to hand-rear them
If the baby has been rejected, then you’ll need to feed it every 1 - 1.5 hours, giving them as much as they want each sitting. If the baby has been completely rejected and is getting no maternal care, then you’ll need to keep them at a good temperature with a hot water bottle wrapped in blankets (so it’s not too warm). You’ll also need to encourage them to go to the toilet for the first three or four days, as they can’t do this without assistance. Once you’ve finished feeding them, use a cotton bud to gently wipe at their genitals to encourage them to poo and urinate.