Does Zakat become obligatory on a sum of money owned by a child who has been in a complete coma for three years, knowing that the amount is essential to cover her daily medical expenses?

Zakat (definition) is obligatory on all zakatable assets (such as the two currencies – gold and silver) if they reach the nisab (definition) (threshold) and a full hawl (lunar year) passes over them, regardless of their owner (whether an adult, minor, male, female, sane, or insane). This is because Zakat is obligatory on the wealth itself. The guardian of the girl must pay Zakat on her wealth at the end of the hawl. Her future need for the money does not prevent the obligation of Zakat. If the wealth is consumed before the hawl or falls below the nisab, then there is no Zakat due on it.