I'm sure we can all related to the separation anxiety we felt when we dropped off our furbabies for their surgery! It's like parents taking their kid to school the first day!! Just like kids, though, our pets have their own coping mechanisms and will be just fine. The neutering part only takes a few minutes, really, and for most of their visit, they'll be sleeping anyway. So, it will go much faster for them than it does for you!
We always got them a new toy when they visited the vet...just a little fuzz ball or bell ball or something...that way they get rewards for being such brave little guys! I'm sure your fellas would appreciate something like that, too.
Be strong, mom! They'll be fine and will be happy to see you at the end of the day! <hugs!>