On our first trip DD was 5.5 and wants to be a wildlife vet, so we made sure to go to planet watch. We saw a medical exam on some rodent I now forget, listened to a CM show us and talk about a blue macaw. There was a scorpion in a clear box another CM was bringing around and we learned a lot. Photos with chip and dale and rafiki.
The train ride was really relaxing. It was worth it but I'd place it below Dinoland as an attraction at that age. Then again you can move more quickly and not watch a vet procedure or listen to the talks, just see everything, pet some goats and take the train back.
Also have to mention the shows, I agree wiht the other poster that lion king is best I've seen on property, and we loved Nemo as well. Last trip we saw Nemo for the first time, right before the park closed. It was wonderful! I have not seen flights of wonder though and I've heard people enjoy that quite a bit too but for the age I think the lion king or Nemo is a better bet.