I've semi-perfected the art of cooking for my two kids "special" diets because of their food allergies. One trick I think that works is to cook recipes from vegetarian cookbooks because there isn't any eggs, dairy, or other artificial additives in the recipes. I just eliminate the nuts from the recipes that call for them. Another thing I did in the beginning was to spend HOURS reading ingredients on store bought food boxes as well as ingredients in regular recipe books or online cooking websites. I investigated the meaning of words such as "albumin" is a word for eggs, "durum" is a word for wheat, and "casein, whey, and lactose" represent dairy.
It can be over-whelming to start any new diet, especially if the child is a picky eater. Whether a parent is starting their child on a gluten-free casein-free diet or any other special diet because of food allergies or due to health reasons (celiac, autism, diabetes, etc.), it can be expensive and over-whelming to know where to start. This link gives a few ideas on how to do it and save some money:
How to begin a GFCF diet on a budget | eHow.com
I wrote this article over a year ago. I would love to have your comments on what worked for you. But I do think any parent of a child on a "special" diet will tell you that it takes alot of time and energy to figure out what the child can safely eat, then after awhile it becomes second nature and isn't too difficult.