Zakat on Land that Granted to Others?

A man granted his children some land, is it obligatory to pay Zakat (obligatory charity) on these grants? If so, should they pay it with arrears as from the date in which they received the land? Or should they only pay Zakat from the date when they became aware of its obligation? And how about Zakat on land has been granted to children who are not yet adults? (find your answer here)...