You might want to read over www.guinealynx.info/handfeeding.html#pups
Cows milk is not suitable for guinea pigs. I would switch over to only critical care. I understand their bums need to be wiped to stimulate defecation. As for populating their digestive tract, if you can get hold of some of the cecal feces (special poops that are reingested - greenish and smelly), that would be your best bet.
Usually babies are able to eat solid food within hours of birth. Regular (not softened) pellets and hay should be available at all times now. Do you have an adult you can put them with? So the adult can model behavior.
- And got the T-shirt
If you're ever faced with this situation again, goat's milk is the closest to guinea pig milk of any that are available. Guinea pigs are not engineered to process sugar, and sweetened condensed milk is more sugar than milk. If it were me, I'd discontinue that immediately.
Marumoto products can be bought via her online Marumoto/Maru-vet shop. Also some Polish online pet food and accessories shops carry it, like Superświstak ("super marmot:)".