I agree with word factory, that many children start lots of hobbies when they're little, but as they get older, the commitments for all the hobbies naturally increases. So, children have to make a decision about which ones they want to commit to. And the others fall by the wayside.
My DD started gym, 1 hour a week, ballet tap and modern1 hour a week and swimming 1/2 hour a week. Over the years this has increased - gym is now at 18 hours, ballet has become 2 hours (but that includes dropping the tap) and swimming has fallen by the wayside.
Even if she didn't compete at gym, the hours naturally increase. They start with a half hour lesson, but this can turn into a two hour plus lesson when they're older.
As children get older, and better at their hobbies, the demands upon them increases and they have to choose. My daughter also learns the flute (she's year 5). It's manageable at the moment, as she doesn't get a lot of homework. But when she goes to secondary and her homework increases, she'll have to make decisions about which of the three hobbies (gym / ballet / flute) she wants to commit to and which ones she'll give up. Unfortunately, you can't do all the extra curricular stuff with all the extra homework at secondary school.
Personally, I'd prefer to see less homework, and more children doing extra curricular like music. And I'm a teacher too
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