I just want to say ditto on a few things - full genetic sequencing is the very best option...yes there are over 1500 genes and as Stacy said testing you for only 25 is inadequate. Sweat tests can definately be misleading, many people with normal sweat test numbers have genetically confirmed cases. Yes, again there are mild cases and people being diagnosed in their adult years -- the term "mild" catches a lot of flack for other reasons, but for the purpose of what you are asking/trying to determine, the answer is yes - it is entirely possible one or both of your kids have a "mild" case of CF.
One of the key points you want to make in your argument for pushing full genetic testing is that they are both displaying CF symptoms. You will probably have to settle for sweat tests first and possibly only genetic screenings of 97 genes or less....just keep asking for complete genetic sequencing... change doctors if you have to...getting in touch with a Certified CF clinic is best too. To find the closest clinic to you go to cff.org
Best wishes, and please keep us posted on what you find out.