We started a little over a week ago -- though not for the first time, but DD did start K. Sounds like you are covered on the "things to buy" for the moment. Though be prepared for the "X has this cool snack container / thermos / pencil case... Can I have one too? routine.
A few tips:
Get into the habit of having each kid set out ALL of their uniform pieces the night before. We have hooks on the end of the bookcases for this. Include skirt/pants, top, belt, socks, shoes, jacket if needed, hair bows, under-skirt shorts -- whatever is part of getting dressed. This gives you time to fix "I can't find this or I need a this clean" problems. There is less flexibility in some uniform requirements, so you need recovery time if there is an issue. We do this as part of our bedtime routine.
I pack snacks/lunches in the morning just because I don't have space in the fridge for lunch boxes and I don't like pre-made sandiwches. But if you can do stuff ahead, great. You will probably need to experiment for a bit before you have the right amount of lunch and snacks. Both my kids eat a surprising amount during thed day.
Have a specific place to store backpacks when they come in each day. Have a routine to check/empty them (otherwise you'll find that uneaten apple 2 weeks from now...). We have a set of wire "cubes" (think cubbies) for stuff like this by the front door. Also make it part of your routine to read and sign things immediately and put them back in the packback. Otherwise stuff gets lost and/or you don't know about something important. No fun to scramble for a permission slip in the car the day its due! This is especially true the first couple of weeks of school when lots of info comes home!
Be prepared for tired, tantrummy, whiny kids the first few days. Its an adjustment (even if they are used to school). It doesn't mean you made the wrong choice (I know this has been agonizing for you), it just means everyone needs to adjust to a new situation. Even my kds (who are used to school, and this school) have issues during the first few weeks of school. Transitions take time -- don't jump to conclusions too fast!!