Sometimes, it seems as if a child is not in love with an instrument, but it could be that it is because of many other reasons - the teaching not inspiring, the pieces too repetitive or boring or not "fun" for the younger audience, pieces not challenging enough, student's technique not strong enough to keep up with the pieces, student not being held to high standards by the teacher (my son used to think that piano was "too easy" when we had one such teacher) etc.
In our case, my son wanted to try another instrument when his abilities had outgrown the teaching ability of his piano teacher. I did not know enough at that time to notice that. He likes to work on challenging pieces and he was given a lot of cutesy pieces and he told himself that piano was too easy for him. I started him on his second instrument as well as found a classical piano teacher (his favorite style) who set very high expectations for him. He loves his second instrument, but seems to have regained his love for piano again because of the structure imposed on him by the new teacher. He also learned theory, sight reading, ear training, composition etc and this helped him read and play music more efficiently. I sit with him to help him practice most of the days so that practice is not lonely and isolating. I periodically take DS to the local music store and let him pick books of his choice and request his piano teacher to incorporate his favorite pieces into her lesson plans. She even lets him play his pieces of choice for his recitals and he loves the ability to choose some of what he plays. Hope this helps.
