Hi J.,
If you are not familiar with Sensory Processing Disorder, please look it up on the internet and/or look for the book THE OUT OF SYNC CHILD, by Carol Kranowitz. SPD can co-exist with ADHD or it can look so much like ADHD that it is misdiagnosed!!
Your daughter my have perfectly normal hearing, but may not be processing what she hears..which is why she may appear to be "in her own little world" and have the comprehension difficulties you discribe. Often if one child has sensory issues their siblings will also have issues (altho' they may be completely different issues)...so perhaps your son's negativity is really perseverations---which is also a part of Sensory Processing Disorder (SPD)
You can have your children screened my a good Occupational Therapist to see if there are areas of weakness which can be strengthened thru therapy. While Pediatritians are becoming more knowledgeable in this area, they often do not have as much information as is necessary to completely diagnose.
The good news is that is with simple, FUN, activities (therapy) your children's neurological systems can be stimulated and can become stronger! The younger they are diagnosed and begin therapy, the easier their academic and social lives will be.
Please explore this issue in depth. It is very easy to look at the extreme examples that are described in books and on websites and think that SPD does not apply to your child, but, as you read, remember that SPD is a spectrum disorder which can vary greatly from individual to individual and can even vary from day to day for the individual child!!!
If I can help or share more information, please email me! I would be happy to answer any questions you may have...or help you find someone who can!
Good luck!
V.