If mom is feeding them, she'll feed them all. She just hovers over the nest and the babies will feed from underneath her. Normally they all get fed the same, but sometimes one or two smaller ones won't be able to feed as well as the bigger babies.
In that case, you can take the big ones out of the nest, just leaving the smaller ones. Then put mom in the nest and give her some treats/pets to keep her calm. Try to get her to stay for about 5 minutes then let her leave. Then put the big ones back in the nest.
Don't be scared of moving the fur and hay aside to look at the babies or reaching in and moving things around a bit. It's more important to be 100% sure nothing is in the nest that shouldn't be than to keep the nest exactly as mom made it.
Under the layer of fur/hay there should be a fist sized deppression where the babies will all be nestled together.
The dead baby could be one that was born after the other ones or a baby that was latched on to mom's teat when she jumped out of the nest and died of exposure. Is there a lip on the nest box? A lip will help keep babies in the nest, it will pull them off a nipple as mom jumps out of the box so they don't end up on the outside of the nest box.
Mom WILL NOT move the babies, so it's important to check for babies out of the nest a couple times a day.
--Dawn