I am no expert. This chart might help
http://www.oema.net/lexiles/ReadingLevelComps.pdf. I can see where she might be going, balancing what might be developmentally appropriate versus readablility levels. The problem is that you should be able to do better than Magic Tree House. Does you DS have a problem with inferance meaning, problems with fiction, or something similar that she might be basing this suggestion on? Is she negating the DRA results because most school districts require written answers above a certain level? The books she mentioned are on track based on Lexile level, but not necessarily by DRA level.
You might do better to visit
www.lexile.com. In book search you can set both your age range and your lexile range. If you use the higher lexile around 500, which gives you a range higher and lower you can find tons of books that might be much more appropriate for a 1st grader by age but at a higher reading level. You can also limit by topics of interest. I would play around with that using the chart and see what you can find that might be of interest.
