We have music schools over here that offer a program called KindiMusic. It's really good because all the instruments are there and the children get to swap and play with them all in a fun and relaxed manner. Are there any programs available like that in your area?
The concept is great because there is no pressure and the child can explore different instruments. If you see a leaning towards a specific instrument you could persue it, otherwise if it's a passing phase, you haven't lost out on making an instrumental investment.