Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.009.

The guardian of a child or of a mentally retarded person must pay zakah on his behalf from his property if it constitutes a nisab.

‘Amr ibn Shu’aib reported from his father backed up by a chain of sources going back to ‘Abdullah ibn ‘Amr that the Messenger of Allah, upon whom be peace, said: “One who becomes the guardian of an orphan with property must trade on his behalf and not leave it passive in order to avoid depletion of the property by sadaqah.”

However, this hadith has a weak link. Still, al-Hafiz affirms that: “There is a similar hadith of the mursal type in the compilation of ash-Shaf’i, who confirmed that it is considered a sound one. ‘Aishah used to set aside zakah for the orphans who were under her protection.”

At-Tirmizhi concludes that: “Jurists differ on this issue. More than one companion of the Prophet, upon whom be peace, said that zakah may be taken from an orphan’s property. Among these are: ‘Umar, ‘Ali, ‘Aishah, and Ibn ‘Umar. This view is also supported by Malik, ash-Shaf’i, Ahmad, and Ishaq. Another group, including Sufyan and Ibn al-Mubarak, hold that: ‘Zakah should not be taken out of an orphan’s property.’ “