Sibelius does have a lite version - still costs £100 or so - which is very powerful, but a warning: its user interface is spectacularly unintuitive. We bought it for DS's 10th or possibly even 9th birthday but it's only now at 11 that he's really getting into it. At school they use Noteflight, which DS says is less flexible but "quicker" by which I think he means more usable. With hindsight, I think we should have gone for Noteflight at home too.

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