Is the granddaughter's inability to pay back the 250 she took from her grandmother's money, which the grandmother manages for her, considered a dereliction of trust?

There is no blame upon you for taking money if you are in need. A guardian of a person's wealth is permitted to take from it what suffices their needs, in a reasonable manner, just as Allah permitted the poor guardian of an orphan to partake of the orphan's wealth in a reasonable way. It is preferable to agree with your maternal uncles on a fixed monthly sum. This taking must be limited only to what suffices your needs, because it is not permissible to dispose of another person's wealth except with their permission, as the Prophet, peace and blessings be upon him, said: "Indeed, your blood, your wealth, and your honor are sacred among you..." and "The wealth of a Muslim person is not lawful except with their willing consent."