I use lunchbags from PBK that can also fit a thermos in the top if I want to send a hot lunch. Their lunches are not in the fridge so I include a gel pack as well. I put everything in Lock & Lock plastic containers so I don't waste bags. She gets her drink and silverware/plate etc. at school.
As for a backpack, we have a small one my DD carries her loveys in. Other than that, she doesn't need to bring anything to school (she carries her lunch box separately). Her extra clothes all stay at school. I would definitely suggest a pre-school size backpack. I tried a regular one on her the other day and it was too big.
I would check with the school and find out their requirements first. My kids go to public pre-k and they must have a full-size backpack (to fit a 1" ring binder and/or folder). I use a 2-compartment lunchbox just like the one shown above and pack it about the same way. I often include an icepack if the fruit is something that needs to stay cold (like melons).
Truthfully, DD1 doesn't even like to use her backpack. She was in 3K and the novelty of the backpack wore off this spring. She hated carrying it (even though her other friends still used theirs). I usually got projects in the morning so she didn't bring anything home except her library books once a month.
We don't have lunch at school (yet) so I have no input on that.
I would wait to find out what the school recommends as well. I had bought DS a small backpack when he first went to pre-k at 3 and was told he did not need it. Extra clothing was kept at the school in a plastic bag in a bin with his name on it. The same went for the 4s program. The only day I sent him with a backpack was Mondays when he went to swimming lessons after school. As for his lunchbox, we have a Cars one similar to the Barbie one that Ohana posted. They gave us a sheet w/ suggestions on it. One of which was to include a cold pack.
I'd also wait and see what the school recommends. They may not allow outside lunches, they may or may not refrigerate lunches, and they may have restrictions on backpack size. You could try calling to ask now, most schools have supply lists ahead of time so if someone is there to take your call they should be able to get you the supply list.
Thanks for all the help. I don't think I've ever seen bento boxes around here, I love them. The preschool (DD preschool) is at a local elementary school that I've worked at, so I am familiar with a few things. There is no way I'd let him eat lunches there. There's no way I'd eat lunch there. Ick! I have never seen a school around here not refrigerate lunches here either, but being preschool, it might be different. I'm planning on them not. Because they are out for the summer, I'm really unable to get hardly any info until the fall, (he won't even have his IEP written until less than 15 days before he starts) and that's making the planner in me twitch.
I hear you. When DS1 started preschool at a local school, the director said the teachers would send us a letter with the requirements, and they didn't send them until about two weeks before school started. So annoying!
isign If there's a Daiso or Asian grocery near you, you might be able to find bento boxes locally. I know of several stores in my area that sell them, but I live in an area with many Asian immigrants. You can also use a regular tupperware type container and google bento for ideas on how to pack lunches for your LO.