The religious leaders were indignant toward Jesus. They were offended that children would honor Him.  But Jesus replied to them by quoting Old Testament words that David wrote in the Psalms.

Psalm 8:

14 The blind and the lame came to him in the Temple, and he healed them. 15 The leading priests and the teachers of religious law saw these wonderful miracles and heard even the children in the Temple shouting, “Praise God for the Son of David.”

But the leaders were indignant. 16 They asked Jesus, “Do you hear what these children are saying?”

“Yes,” Jesus replied. “Haven’t you ever read the Scriptures? For they say, ‘You have taught children and infants to give you praise.’ “