Should I be considered sinful for having eaten the teacher's candy that I was asked to bring to school?

If you do not remember returning the candy bag, it is a trust in your possession. A trust is only guaranteed (i.e., its loss is covered) if there is negligence or transgression. If it was damaged without negligence on your part, then you are not liable for its guarantee.

If your family's insistence was merely a strong plea, and you ate it due to your own negligence, then you are obliged to guarantee it. However, if they ate it forcefully and you were unable to prevent them, then they are liable for its guarantee.

The right must be restored to its owner. If the owner cannot be reached after searching, then its value should be given as charity on their behalf. If the owner is found later, they are given the choice between accepting the charity or reclaiming their right.

If the trust was taken before puberty, there is no sin upon you, even with negligence, because the pen is lifted from the child until they reach puberty.