Here's a vote for the xylophone. One thing all kids do is hit things. With a good xylophone [at least 2 octaves], there will normally be combinations of sound. It's fully chromatic and uses the same 'keyboard' as the piano. If the child has any interest at all, melodies will develop naturally without the need of teaching. Then, when the time to teach arrives, transition is easy.

In addition, it can be sold for about what you paid for it if the child outgrows it.