General anaesthesia is performed on an empty stomach. Though, the two vets that I have been to ask that the cats be fasted for a minimum of 12 hours. But again, check with your vet.
The helmets aren't necessary. Your kitties will want to lick their stitches. Keep an eye on them, just to make sure they're not actually pulling them out. It's not a good idea to use bitter apple on the area given that it has alcohol in it. Alcohol can cause a skin irritation at worse and a lot more stinging around the incision. Also with the stitches, see if your vet is able to use dissolving stitches. This saves taking the kitties back to get the stitches removed, although, the vet will ask for a check up about a week after the procudure is done.
If you're worried about the pain levels for your kitties. And I won't lie about this, they will be in pain. You can talk to your vet about medication to help manage the pain. And if you do choose this option, have the vet show you how to medicate the cats. There is a thread with suggestions but since they've just had an operation, there may be things that just aren't appropriate to try.How to medicate a cat Thread
Ok...when it comes to feeding when you bring them home. Yes, by this stage they are feeling very hungry. Give them a small amount at a time, not one large meal even though it is very tempting to do so. This will prevent your kitties gorging themselves silly and possibly vomiting later. You may want to just give them dry food. It's less rich in nutrients, so less likely to cause an stomach upset. And then the following day, introduce wet food gradually. They should be eating normally by the second or third day after the operation. It really depends on how they recover. Some cats recover sooner.
They may want to cuddle up to you during this time for comfort. Ideally, it's best to let them climb onto you. Get down to their level on the floor. Try to prevent them jumping and running around, though with some cats this is impossible to do. My male cat Russell came home and decided to climb a staircase at full pelt. And then meowed to be carried down because it hurt too much to come down.
If you do pick them up, do so very gently and definitely with two hands to prevent unneccessary movement at the incision area. Your kittens will let you know if you do something that hurts them. Ask your vet to show you the best practical way to pick up your cats. But remember, when dealing with any animal in pain, there is always a risk of them lashing out. If either kitten lashes out, remember, they're in pain and not to tell them off. Try to comfort them with your voice. And if they let you, a gentle pat or two.
And the most important thing of all is you! Relax! Don't stress out over it. Your kittens will pick up on that. If you have any concerns, talk about them, whether it be your vet or us. Good luck!