The infusion sets that you insert at 90 degrees have an option between 6mm and 9mm. I use the 6mm and find that the 9mm caused pain or problems with kinking for the plastic infusion sets. (I use 6mm Sure-T which is a metal infusion set that I actually find to be much more comfortable than the plastic ones because it is thinner.)
There are also infusion sets that you insert at an angle (examples: Silhouette/Comfort, Inset 30, etc). These infusion sets are much longer, but don't go in deeper because of the angle of insertion.
The infusion set names vary by brand. If you do start on the pump, I recommend asking for samples of ALL the infusion sets and let your daughter decide which is most comfortable. Some people use one type of infusion set in the rear end, a different type of their stomach/ arms / thighs.
Also, many parents use EMLA numbing cream before inserting the infusion set. As an adult, I don't find the insertion painful (rather it is only rarely painful when I hit a nerve, but then it does hurt). But when I was seven, I'm sure I would have hated it. So numbing cream could be a good option no matter what type of infusion set you choose.