(Just testing out the video-function on Blogger with a quick little video of our rex-mumma pig 'Subi' feeding her newborn daughter 'Bonnie'.
When babies suckle from their mums over the first few days/week or so they poke their little bottoms in the air and mumma pig licks them clean. This is because the first poos are quite runny and sticky - a little like human babies and their meconium poo. After the baby-pigs have been eating solid food for a day or so, they'll start to do tiny little solid poos and gradually the mum wont have to clean them up like this!
It's important to note that if you're hand-feeding very new baby guinea pigs you will need to take over this aspect of the cleaning as well. A gently wipe over their bottom a few times with a wet cotton ball should do the trick. It will encourage them to open their bowls and get their digestive tract moving too).