A typical depiction of a disordered child's playroom, often said to, supposedly, give a visually depict the second law of thermodynamics, that a system left to itself will tend towards disorder or that entropy will tend to increase in an isolated system.
In thermodynamics, entropy (child’s playroom) refers to a popular way of describing entropy as a tendency of systems to move towards disorder (or chaos), just as a child left alone in an initially organized room, given time, will move the room to state of disorder. This is sometimes referred to as the entropy messy room model.

American mathematician turned journalist Charles Seife claims that nine out of ten high school physics teachers, when asked, will describe entropy as the measure of the messiness of one’s room or how poorly one has arranged books on one’s shelf. [6]

History
The child’s playroom model of entropy seems to have originated in the 1961 lecture “Entropy and the Unity of Knowledge” by American physicist Peter Landsberg, where he states: [1]

“Tidy way all your children’s toys in a toy cupboard, and the probability of finding part of a toy in a cubic centimeter is highly peaked in the region of the cupboard. Release a randomized influence in the form of an untidy child, and the distribution for the system will soon spread.”

In 1971, American art theorist Rudolf Arnheim referred to this as the “child’s playroom” explanation of the principle of disorder as advocated by German physicist Max Planck, who in his 1915 lectures seems to have given the impression that the end state of certain material systems is one of maximal disorder, as advocated previously in his principle of elementary disorder. [2] The following definition of entropy, to cite one example, comes from the chemistry department of Boise State University: [3]

“Entropy is the measure of disorder or randomness in a system. It is a little like a child’s playroom. It takes a lot of energy to put the toys away…ordered. It takes a child a millisecond to achieve total randomness…entropy at work!”

 Figure 2-37, entitled "An Everyday Illustration of the Spontaneous Drive Towards Disorder", from the 1994 Molecular Biology of the Cell, captioned as "Reversing this tendency toward disorder requires an intentional effort and an input of energy: it is not spontaneous. In fact, from the second law of thermodynamics, we can be certain that the human intervention required will release enough heat to the environment to more than compensate for the reordering of the items in this room." [5]

Difficulties
There are some who argue that this model has nothing to do with entropy or entropy increase. This may or may not be true.

In American chemist Frank Lambert’s 1999 article “Shuffled Cards, Messy Desks, and Disorderly Dorm Rooms”, he argues that the shuffling cards model of entropy and the messy room model of entropy are nonsense examples of entropy increase. [4]

It is true, according to German physicist Rudolf Clausius, that entropy change in a cycle has to do with the energy associated with the work the molecules of the system do on each other during the course of time between which the system returns to its near original state. In this sense, the work that two human molecules, the child and parent, do on each other in playing in the room and later cleaning up the room can is related to entropy change. The loss of energy involved in re-tidying the room would thus be considered an uncompensated transformation. This, however, is an involved analysis. The original Landsberg definition, to note, seems to be based on the Boltzmann-Planck, S = k log W, interpretation of entropy as a function of quantum states, and not precisely that defined originally by Clausius.