The first time we took a day from Disney to visit wizarding world we purchased our tickets at the park and had no problems doing it that way. We arrived at the parking garage about an hour before park opening and had plenty of time to purchase tickets, get into the park, and to the "rope drop" before they allowed us all in. If you can easily purchase tickets ahead of time then great, but you shouldn't have any problems waiting till you get to the park if that works better for you.

I agree with what Karen said about the later hours. We always head to forbidden journey first thing in the morning, but have found the lines to be just as short the last hour or so that the park is open.

If your daughter is a HP fan then I think it will be totally worth giving up a day in Disney to experience wizarding world. It's been really well done and a must for any potter fan!