What is best to see at Ellis Island and Liberty Island?

am going on a field trip with my daughter. I am a chaparone. Once we get to the Jersey peir, we are on our own. The school said that we can get on the ferry, go to ellis island, then liberty island, and then back to the Jersey peir to board the bus. We have limited time for each island (because of a four/five hour bus ride) and was wondering what is a must see for the kids, what they would most enjoy, and what is best educationally? They are in sixth grade and going as part of their immigration unit in school. What information should we know about the islands? Anything you could share would be appreciated.