I went to both on my trip last year and have decided to only return to Typhoon Lagoon this year. For me Blizzard Beach was a little scary I went on the Slush Gusher and that totally put me off, I almost drowned. For me the slides there weren't 'fun' and I'm an adult. Obviously I don't have experience of the kids area, I'm not a parent and the youngest in our party was my sister of 13. It did look pretty good however I think I've remember hearing people saying in that past that they would walk their child to one of the slides, the child would go down but there wouldn't be an easy way for the parent to get to the end of the slide quickly. So the child would be left unattended.
Typhoon Lagoon however I thought was brilliant, much more family friendly in my opinion. Absolutely loved the family raft rides, which I thought were pretty gentle, Castaway Creek was excellent - never too crowded and not too deep, even Crush'n'Gusher wasn't too bad and more than one can go together at one time so you can always keep watch. I will however agree the wave pool is a no, no - too forceful. I did again walk past the kids area and it looked really sweet, I'm sure your DD would enjoy it and because the other slides and the Shark Reef at Typhoon Lagoon are relatively laid back she'd probably love a couple of the more relaxed ones too.