Today is Palm Sunday, a day that commemorates Jesus's "triumphal entry" into Jerusalem. This was an incredibly subversive act, both politically and religiously--a bit of prophetic performance art--as Jesus and his followers reenacted the ancient Jewish ritual of the king's enthronement (for which Psalm 118 was written and used). But, as biblical scholar James Sanders points out, in the case of Jesus, "The messiah has arrived and been acclaimed king. He has been recognized as king by acclamation not from those with power or authority but by a rather scragly crowd of disciples and followers."
The participants shouted "Hosanna!" which was a cry to God for justice and mercy. "Hosanna" was what a person would cry out to the judge when they came into court, a reminder to the judge to be just and fair and merciful in hearing their case. At the triumphal entry, the people were calling out to God to hear their case against the oppressive religious/civil/economic system that they were under.
Sanders says, "This enactment of the psalm  as a prophetic symbolic act would have been no less blasphemous and scandalous to those responsible for Israel's traditions (and they would have known them well) than similar symbolic acts performed by the prophets in the late Iron Age [such as Isaiah, Jeremiah and Ezekiel]."
So, if you go to church today and see the children waving palm fronds, consider that what they are reenacting is a moment of bold prophetic civil disobedience against rulers and powers and authorities and systems of oppression and the audacious proclamation of a different king and kingdom--a rule and reign marked by mercy, fairness, truth, compassion, kindness, peace, inclusion, grace. A kingdom of love. The kingdom of God.