My seven-year-old daughter only ever wants to wear skirts, dresses, or sweat pants.  Most of the time, this is just fine, but there are occasions when jeans or slacks are really the most appropriate item to wear  (particularly field experiences and times when we will be doing activities where we get down on our hands and knees in the dirt).  

Besides, she is growing up, looks good in jeans and slacks and it is just time for her wardrobe to expand in this direction.  

How can I make sure she is as comfortabe as possible so she can enjoy these other types of clothes for when she does need to wear them?