I am also making a sweatshirt for my niece (I'm almost finished!) and added a i-top brad I made with fabric to the center of a fabric flower and to the zipper. Even though the brad is on the zipper pretty securely, I think I'm going to sweet talk my father-in-law into showing me how to solder them together so the i-top brad wont fall off.
Here's a close up of the flower, I just poked the i-top brad through the center of the flower and then opened the backing up and pushed it down into the backside of the flower.
(Sorry the color is off quite a bit in these pics, I took them at night)
For the zipper pull, I just put the i-top brad through the center of the zipper pull and then tweaked the backing so that it wrapped around the zipper pull and tucked the ends behind the head of the brad.