What is the children's hour?

The Children's Hour is that time in the evening when the sun is setting. The father is working by lamplight in his study and hears the pattering of footsteps and soft voices of his three daughters. Then there is then silence, but he can see in the gleam of their eyes as they descend the stairs that they have mischief in mind. Suddenly they burst into the study, which he compares to a raid on a castle wall, and surround him. They climb his chair and cover him with kisses. He addresses them, asking if they think they are a match for an old man such as he. He now has them and will not let them go; they will stay in "the dungeon / in the round-tower of my heart" forever.