When you look at the babies, are the bellies round? Like they swallowed a ping pong ball?
If they are not being fed, they will be all wrinkled and shrivelled.
Can you post pictures of the babies? Most of the time, from looking at a picture, you can tell if the babies are being fed.
It is very normal to never see mom in the nest with the babies. It only takes about 5 minutes to feed and clean the babies, so she will hop in and out in no time. She is probably waiting until no one is around so she feels safe.
The babies should try to feed when placed near mom's nipples. They are greedy little buggers.
Also, if you stick you hand in the nest, the babies should have lots of energy and start jumping around like little jumping beans. If they have lots of energy, then they should be getting enough food.
--Dawn