Well it's not easy to explain Einstein's Special Theory of Relativity to kids but here's a video where the theory is explained in a cool way

The steps Einstein took to develop his special theory of relativity are as follows: First, Einstein tried to understand what must be implied by the fact that light travels at a speed independent of its source. From this, he realized that time and space are not simply two unrelated concepts; rather, they must be linked in a space-time system whose properties appear most dramatically when objects move very fast and are observed by two different observers moving with respect to one another. His final and greatest step led him to discover and clarify mathematically many new things about the world that are important in each and every frame of reference.