I've mentioned before on asthma threads that learning to play the flute was hugely beneficial for dd2's asthma (she used to visit asthma clinic regularly and her peak flow got higher and higher as she got more proficient) and there was a whole page about it in the Graun yesterday (here ).
Definitely worth considering for older children - she started at 7 and the significant increase in lung capacity and improvement in breath control came later.