I’ll bet you the teachers and staff get to eat the goodies after they steal them from little kids.
No, they don't. The policy specifically requires the food to be returned to the kids at the end of the day, and because of that there'd be serious consequences if it wasn't.
If that wasn't in the policy, legally, under the law as it applies in Australia, the food could be destroyed (a teacher can't permanently deprive a student of an object of significant value, but snack foods would not come into that category). If the school has stated such items will be confiscated and destroyed, they can be.
Whether that includes eating them, I'm unsure legally.
In Australia, teachers in state schools are agents of the Crown, and have considerable power over children because the Crown's powers are almost unlimited except where they have been expressly limited by law. Teachers in private schools derive their powers from the doctrine of in loco parentis and except where powers are limited by law, have pretty much the same powers over a student as a reasonable parent does. Under both these principles, the legal right to take food that is considered unhealthy is a minor issue.