Hi Felicia, Great question!
My first thought was to recommend Typhoon Lagoon, because Blizzard Beach has so many stairs to climb to get to the cool water slides. You can take the chair lift to some of them, but the line gets long for that. As a whole, my family prefers Typhoon Lagoon.
However, because you have a 6 year old, I would actually recommend Blizzard Beach instead. In fact, we were there just last week with our 6 year old daughter. Her favorite area of the Park was the
Ski Patrol Training Camp. She was too big for the
Tikes Peak area, but found the Training Camp to be perfect for her and lots of fun. Typhoon Lagoon seems to lack an area for the "in-between" kids. Most 6 year old children find
Ketchakiddee Creek to be too babyish for them.
I also have a 10 year old. I asked him for his suggestion and he thinks your oldest daughter will like either of the Parks and will find numerous fun slides at both of them...and he's right! Since she'll be happy either way, I'd base my decision on which one has the most for your youngest daughter too. That is Blizzard Beach in my opinion.
Hope this helps and have a great time,
Kaylene