Consider that four people can cross the river for the first time. Then it takes 1 person to bring the boat back. So a round trip can take three people. So we need 19/3=6 (times) ... 1 (people). But for the last time, four people can cross the river for the last time without bringing the boat back. Extra people can accompany him to cross the river for the last time.

So it takes six round trips to send all the children across the river.