My 7 year old is still not tall enough for Dragon Challenge, and only started doing Forbidden Journey on our 2nd visit, but despite that, it was still worth going. During our first two trips, no one did *any* bigger ride other than Forbidden Journey, because DS is too small, I was too chicken, and DH can't quite get on the thrill rides yet, but our LONG stays were still worth it!
Just being in Wizarding World, for a Harry Potter fan, is AMAZING, even if you don't ride anything in there. (but there's Hippogriff and there are other things to do in the parks, so don't just walk around WW and leave, OK?)