Should I buy a backless booster for field trip kids?

I'm driving during DS1's field trip on Thursday. DS1 and DS2 will be in our car, but we have a Mazda5 which means I'll be able to take 2 additional kids with me. It's a second grade field trip (7 year olds) and DBF drove on DS's first grade field trip last year and not a single kid had a booster (even though notes were sent home saying to bring them). CA law is that kids can be without any kind of restraint once they are 6 years old OR 60lbs, so pretty much no one past 6 years old has any kind of carseat or booster from what I've seen at school. I already have a Monterey so I'm thinking I should put that in the car, but that leaves one seat without a booster. Should I go out an buy a cheap backless booster? I really don't see any of these 7 year olds being able to pass the 5 step test and it would really bother me to put a kid in my car with the seatbelt not fitting properly. Would the Harmony Literider be the best and most inexpensive choice? I see it's only $13.50 at Walmart. I have a Target right down the street from me but I think the cheapest option there is the $20 Turbobooster right? Walmart is 5 miles further from me. DS1 will be riding in his FR85.

I would if it doesn't tax your budget - it'll ensure that everyone in your vehicle is seated safely, plus give you a lightweight booster for DS for playdates/other field trips.

Yeah. Get a cheap backless booster. Its always handy to have around.

Yes, I'd get one. And yes, the Harmony ay Walmart is the cheapest. Target might have a Cosco for $15, but I can't remember.