I'm really surprised at the large numbers here. Many of your kids are spending longer on a single activity than the number of waking non-school non-travelling hours my kid has in total!
DS10 spends about 7 or 8 hours a week between 2 instruments, including lessons, ensemble and time snatched in school breaks. Not sure I'd describe him as really committed in the sense meant here; he's keen and conscientious and good for his age (but not the way he is in maths), but not straining to spend all the time he can on music; he still wants time for maths, computing, and games!
FWIW, I think if ultramarina's son were mine, I'd allow him to do the amount of chess he's asking for to the extent that its impact on other family members was acceptable, but I'd try really, really hard not to get invested in his success myself, and not to assume he'd want to do X this year because he was doing it last year - make sure the way is clear for other interests to rise up if they will.
