All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is no Zakat on this piece of land which you bought in order to build a house for your children on.
However, if you purchased this land for the purpose of trading with it and gain a profit from it, then it is considered as a tradeable item, in which case you are obliged to pay Zakat on it for all the previous years.
Allaah Knows best.