I've hand-fed guinea pigs, but they weren't babies so I don't know if I can help you. I used a 1 CC syringe which you can get from a drugstore and just remove the needle.
For one of my pigs, I put a towel on my lap, then put the guinea pig on his back at a slight slope. Then I folded the towel over to cover his belly and keep him warm and left his head sticking out. Then I held his head firm and depressed the syringe slowly. As long as they are making a chewing motion, they are swallowing. I gave him little breaks now and then and recorded how much he ate and weighed him several times a day.
With my other pig, I made him go in his cozy and folded the cozy back until just his head was sticking out. This 'pocket' made him feel more secure and he struggled less. His body was in a vertical position inside the cozy which made it less likely that he get fluid in his lungs. All I had to do was hold is head rather than worry about his body moving.
You could try putting the babies in a sock or some other type of pocket. It might make it much easier to feed them. Good luck!