Carnival and Royal Caribbean would provide the best experence for younger children. Realize that the wildlife in Alaska is truly wild. She is not going to be petting any bear cubs. Also, most excursions do not guarantee wildlife sightings (in 1999, when I went, there was one that did, or you received a refund).
That said, given her interest, she would very likely enjoy the cruise. However, the excursions that offer the best chance to see wildlife can be a bit pricey. She might enjoy the dog sled ride in Skagway. Of course she can pet the dogs. However, the transportation to and from the dog sled camp is via helicopter, which makes the excursion expensive. However, next time I go, I am going to book that one. Also, I believe you have a better chance of seeing a variety of wildlife (bears, eagles, whales ...) on a one way cruise (north or south bound) than on a round trip (out of Vancouver or Seattle). However, the one way cruises cost more.
The best weather is likely to be June and July. In 1999 we went at the end of July for 14 days and only had a little rain on the first day (which was a day at sea). The rest of the cruise was all sun. However, that is the exception. Most Alaska cruises involve clouds and some rain.
If you have the time (and money) you can take a pre or post cruise trip into Denali National Park.
