When it comes to learning a musical instrument, learning to play the piano is without a doubt one of the most beneficial instruments to learn that will teach beginners the basics of musical theory.
Learning important aspects of music like octaves, scales, and chords are key to learning the piano and transfer over to any other musical instrument your child may learn in the future.
One of the many benefits of kids learning music on a piano is the keys are all laid out in a logical, linear fashion. This makes concepts easier to grasp than on the guitar where you would need to jump around to different frets on different strings to learn the same concepts. The next easiest instrument might be getting your child a ukulele.