When the cat's away, the mice will play

When a person in authority is away, those under the person's rule will enjoy their freedom.

Without supervision, people will do as they please, especially in disregarding or breaking rules. For example, As soon as their parents left, the children invited all their friends over—when the cat's away, you know. This expression has been a proverb since about 1600 and is so well known it is often shortened, as in the example.