Final answer:
The question is ambiguous as it lacks cultural or historical context to provide an accurate answer regarding what the children built with collected materials during a holiday.
Step-by-step explanation:
The question asks about a tradition where children gather materials during a holiday and create something in the town's center. This scenario does not correspond exactly to any of the historical contexts provided, such as the Indus Valley cities, Greek architecture, the tower at Jericho, or the anecdote about children guarding against assailants. Thus, without a specific cultural or historical context from the question, it is not possible to determine what the children built.
Typically, facts from history suggest that the Greeks built temples on hills, the Indus Valley had a citadel, and Neolithic settlements like Jericho had towers, but these do not directly answer the question posed.