To stop charges for things other than lunch specify "Lunch only" on your check to the cafeteria; your daughter won;t be able to buy that ice cream afterwards.
There is no way of preventing someone else from using your lunch number; this is THE major flaw in the system. At some (all?) high (other?) schools the picture of the number owner is often (not always) displayed to the register person. In addition, if the owner has specified "lunch only" then the 2ND attempt to use the number will have a higher price associated with it that says it is NOT the first use that day; this is supposed to signal to the register person to notify the manager to confirm identities. Note that, in such a case and if it isn't prevented by the manager, the money removed from the account that day won't simply be a small increment over the price of a lunch, but a MULTIPLE of the price of a lunch.
There is no easy way around the problem of lunch numbers - requiring, say, a "food card" (like a debit card) is pretty much guaranteeing infinite hassle as kids forget to have their cards with them by the thousands every day.
Everything w.r.t. Food Services is centralized in FCPS - other than for trial runs there is no "do it my way" at an individual school.
W.r.t. theft of cash by Food Service personnel - that's a pretty serious accusation. It would be stupid for a food service worker to take a couple bucks from a deposit and potentially lose an important job over it.
You can check how much money is in your (daughter's) account by signing into
https://www.mylunchmoney.com/ - that site not only will tell you how much you have but breaks out every single deduction from your account.