Oh, I was one of those. I lost all sorts of things. I drove my mother crazy. It didn't really help that I found 3/4 of those things again at the end of the year, when I FINALLY cleaned out my desk....
No, you can talk about carelessness, but your daughter may actually have to learn by experience instead. And experience, as they say, is the best teacher.
So don't talk too much about it any more, except to say, "What you lose cannot be replaced by Mommy and Daddy." (And not by Auntie or Grandpa, either.)
If she says, "We can go buy another one," use that not as a lecture opportunity but as a teaching opportunity. Next time you go to that store, take her along. Look at the lunch boxes; see how much they cost. Remember that, at six, she doesn't have a lot of money sense yet. "Ten dollars! Do you have ten dollars?" "No, Mama, but you do." "No, I don't. I have groceries to buy every week. When it comes to your having a lunch box or the family's having food, food wins. Can you earn ten dollars?" "What's that?" "You do jobs and I pay you for your work." "How can I work?" "I could give you jobs at home to do at twenty-five cents apiece. At that rate, you'll need to do forty jobs - actually closer to fifty, because you'll have to pay tax." "But you're my mommy - it's your job to buy me what I need." "I did buy you what you need. You lost what you need."
Don't lecture. Don't yell. Don't try to make her feel bad. You're not punishing her! You're showing her the way the world works. It works that way for you, too.
If she loses her sweatshirt, she can wear something else. If she loses her lunch box, she can use a brown paper bag. It's all right for you to help her look for something for five minutes, if *you* want to, but keeping track of and finding your daughter's things is her job, not yours. Even a six-year-old gets the idea eventually. (At least she's six and not fourteen. There's a lot for her to learn anyhow. If she can remember something before you leave the school parking lot, even though she has to run to the classroom and make you late, maybe you can give her a bit of grace. It's a step in the right direction.)
She won't be able to buy popsicles if she has to save money for a new sweatshirt, can she? That's real-world economics, first-grade level. (Popsicles cost a DOLLAR?)
It's VERY hard for a mother to do! We *want* our children to be happy. But sometimes the way they can learn the things that will make them happy in the long run... is for them to be a little unhappy now, and take the consequences of what they do. As the school year goes on, your girl will probably get the point and start doing a little better.
...That brings back a long-forgotten memory. When my children were growing up, the family clothes budget was next to nothing. We did a lot of thrift-store purchasing, shopping carefully. One year I saved megabucks on warm winter jackets for my two girls. How? Because two jackets just the right size had been left, unclaimed, at the dance school they attended. The director said, "It's been months and not one parent has even called. If your girls can wear them, they can have them." They were expensive jackets! I wonder if the parents of the original owners shelled out for fancy new ones to replace the fancy old ones.