I might be late to this one, my apologies.

My dream was set in a dystopian land and it takes place in a city in the middle of a vast ocean. Because of overpopulation, the city is slowly sinking, so the mayor of the city hatches a plan to decrease the number of birth rates by issuing mass extermination of any citizen who is 13 and younger.

The mayor sends in his secret special forces to raid any homes to find underage children and take them away to camps to be held in until their extermination date. On the day of the extermination, the older siblings of the captured children gather in a crowd to watch (forcibly) as their brothers and sisters are drowned in see-through water tanks. After the screams and pleas for help are engulfed by the water's rush, the swimming tanks are silenced one by one. Their bodies are fished out by a bulldozer, and the older children are free to play in the swimming tanks.

This one, by far, is the most morbid dream I've ever had.