I have posted on this before.
http://www.hawthornevillager.com/phpbb/ ... hp?t=20341
In a nutshell, it is unlikely that cleaning your ducts will have any effect with regards to your child's asthma problems. In fact, it is unlikely that it will have any measurable effect whatsoever, beyond enriching the company that takes your money of course.
It is heartwrenching to bring children to the hospital, and the urge to do something, anything, is very strong. But throwing away money on duct cleaning is really not a good idea.
(also
http://www.hawthornevillager.com/phpbb/ ... t=air+duct)