Complex sentence: However, the kids begin shrinking once they're outside and, once they're each the size of a raindrop, they rain down into a river, which carries them into the town's water purification system.
Split into simpler sentences: However, the kids begin shrinking once they're outside and, once they're each the size of a raindrop, they rain down into a river. The river carries them into the town's water purification system.