Is the sentence right or wrong?
Father: "Children, we'll go to the zoo on Sunday, I promise."
He promised that they will go to the zoo on Sunday.
We use "would" as the past of "will" in the reported speech.
Examples
Anne says she will come → Anne said she would come
I promise I will write → He promised he would write
They are sure they will be on time → They were sure they would be on time