I think it depends, honestly. My children visited AK for the first time at ages 4 (daughter, also a huge princess fan) and 6 (son). They weren't overly interested in the animals (they do love animals, but we have some very nice zoos at home and they've been multiple times), found the AK exhausting to walk due to its enormous footprint, were a little young/scared for Dinosaur or Everest, and had already met all the characters they wanted to (e.g., Mickey, Donald, princesses) at other parks. That left us with two very good live shows and only 3 or 4 attractions (Triceratop Spin, Kali River Rapids, Kilimanjaro Safari, Bug). With a park-hopper ticket, our half-day visit to AK was a no-brainer, but without one, I don't think it would have been worth a full-day ticket.
That being said, our experience probably has as much to do with the ORDER of the parks as anything else. Unless your daughters are super-excited about animals, AK will likely be the least interesting park for them. If we had started with AK and THEN the kids had seen Epcot, HS and MK (in that order), we'd have been building momentum and excitement for the entire trip, with each park offering more things that interested them than the prior one(s). We started with MK however, and after that, everywhere else was a little bit of a letdown for the kids, in terms of not offering as many attractions for all ages, and not providing similar convenience in terms of park layout.
If you're unsure, perhaps you want to show the girls the Planning DVD portions on each park (or check out on-ride videos online) to get a feel for where you think their interests lie. Plan a park for that last day, but keep it flexible, so that you can take a vote the morning-of and see where the family wants to go...