Chances are, your kittens won't miss a beat.
Your vet should give you info about when to remove access to food. Mine was after 5pm the previous day, but others seem to have had 12 hour rules.
I'm assuming you'll get pre-op blood work done? I think it's worth the peace of mind for you!
Care after surgery will depend on whether you bring them home the same day or the next day. I picked Belle and Delilah up the next morning. They were either still a little out of it or just traumatized by the trip to the vet because they kept it pretty low-key the first day. They were stuck to me like glue and slept for most of the day. The next day, however, they were back to their punky selves and I chased them around trying to keep them "still and quite."
I finally gave up and only really stopped them when they started to wrestle.
Watch out for them licking or pulling at their stiches. Belle and Delilah were doing this the first day, but after a couple taps on the chin they got the idea. Keep an eye on the incision - make sure there is no excessive swelling or redness.
Other than that, try not to worry too much. I hated the day without them at home, but was glad when it was done and over with!
for a smooth speuter day!
Oh, and if just Pixel is staying overnight (mine keeps females, sends males home), you might want to send a shirt/blanket with your/Bamf's smells on it to make her more comfortable for her sleep over. Chances are she'll be too out of it to care, but it's nice to know it's there.