Question: How on earth did Annie move Hallie's bed to the rooftop?
Answer: It's likely she has help from the girls that were in her own cabin. As each girl had their own set of friends, they helped Annie and Hallie in whatever pranks they pulled on each other.
Answer: Like my bed in college, the bed could probably come apart so they could stack on each other. They could do that and use a ladder—as someone else suggested—to put the beds on the roof, meaning they wouldn't be as heavy or difficult.
Answer: By climbing on a ladder.
Answer: Even if she had help, four little girls couldn't lift those beds onto the roof.