My dc are those who do music outside school.
The main problem they seem to have had in music lessons at school is that there is generally a core of children (typically boys, but not entirely) who really don't want to do music, and are as disruptive as they can get away with. So they tend to disproportionally do what interests them-pop music, drumming that type of thing.
In dd2's year the whole class learnt an instrument-the same instrument dd2 learnt in private lessons. So she was used as a soloist (to the class accompaniment) and demonstrator. She enjoyed this, and it did challenge her as she didn't want to be found not good enough, so she really worked hard at it.
Both of them have learnt recorders as a whole form, and that seems to have gone down well.
To me, one of the best things you could do is get them reading music on the treble clef particularly. It amazes me the number of people who haven't got a clue, and it really isn't hard, it's the practice you need. I remember teaching myself aged about 6 in an evening.
The music teacher at my dc's school does a lot of extra curriculum stuff for them. There is a lunchtime recorder club for any who want to. A choir (anyone can join, and at times it has about 1/3 the school in) a boys choir (meets over assembly so very popular
), and a small audition choir. There is also an "orchestra", which any instrument can join. There's usually about 6 pianists and several guitarists as well as more common instruments. Basically if they can follow music they can join. It's great because it gives any child who wants to the experience of playing in a group.