We are having a similar problem with our 21 month old daughter. She doesn't hold it in though...she just can't go! The doctor finally put her on a laxative. We're still having some problems. But anyway, before we tried the laxative, the doctor gave me some ideas for her diet....which I will pass on to you. Maybe some of this will help with you. You want to definetely give her pear juice. Pear juice is like a stool softener for kids. Anything with bran in...raisin bran (raisins will help too), bran muffins, broccoli, graham crackers, pears, peaches.
I really wish you luck and hope that some of these foods might ease some of your problems. Take care!