Quantum gravity facts for kids
Quantum gravity is an area of research in physics. Right now, physics is able to describe gravity of reallife sized objects using general relativity. Physicists can also describe the universe on a microscopic scale with quantum mechanics. The goal of quantum gravity is to develop a theory which contains both general relativity and quantum mechanics. However, combining the theories of the big and the small is very difficult in practice.
Physicists are working on a number of ways to write down a theory of quantum gravity. One approach is to search for a unified field theory. Physicists also try to develop theories at an even more fundamental level, like string theory, loop quantum gravity, or other methods, to find out why our current theories of gravity and quantum mechanics cannot be combined.
Images for kids

Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theory

Projection of a Calabi–Yau manifold, one of the ways of compactifying the extra dimensions posited by string theory
See also
In Spanish: Gravedad cuántica para niños