After my husband had cancer last year at age 26, I started researching what was in our food. I read "If It's Not Food, Don't Eat It" by Kelly Hayford, C.N.C. It is a very entertaining and informative book. Everything is backed up by research. I would definitely recommend reading this book to inform yourself on all the of preservatives, sugar, artificial flavors/colorings, chemicals and additives found in food today, and especially foods that are targeted at children. They can really have an effect on behavior, especially in kids who are sensitive to them.
Whether you notice a difference in your daughter's behavior/sleep or not, this book will give you a new perspective on what you put into your body and your children's bodies. This book helped me learn that a lot of what is in "food" today, really isn't food at all and does not belong in the human body. You may not notice the harmful effects right away, and it's what everybody else is eating, so no one thinks there's anything wrong with the way Americans eat. However, these toxic substances build up over time and cause things like cancer, digestive problems, arthritis, headaches, fatigue, obesity, and a weakened immune system which can lead to more common colds and a number of other illnesses. Most of what we eat today can have an effect even 20 years from now.
This book describes ways of eating healthfully that are easy and doable, even for busy moms and people on a budget. I now look much more closely at what I feed my family. It can also teach your children healthy eating habits that they will hopefully use for a lifetime. I also think the idea about the chiropractor sounds like it's worth a try.
Dr. Sears' website has some good articles on kids and nutrition:
http://www.askdrsears.com/html/3/T030800.asp "Feeding Toddlers: 17 Tips for Pleasing the Picky Eater" It has good ideas for introducing new, healthy foods to kids, not just toddlers.
and
http://www.askdrsears.com/html/4/T040200.asp "ABC's of Teaching Nutrition to Kids" I thought this one was especially good because it teaches parents how to make learning about healthy food fun for kids. It talks about how to educate kids so that they make healthy choices on their own later on and so they know which kinds of food are healthy and why. The only thing I didn't agree with was that he suggested rewards, but everything else was pretty good advice.
Best wishes, and let me know if you have any more questions about nutrition. I have a lot of information. C.