our family's experience was that the trundle was horrible.
my (then) 6 year old didn't say anything, because she didn't want to complain because she was "so happy to be in Disney World" but she didn't get any sleep on it at all. It is basically a board, 2 inches off the floor with a shallow "Mattress" on it. She said she almost just climbed off and slept on the floor, I was horrified she didn't just get into bed with us (another family member was in the second queen bed).
She always asks me now when I book, "Mom do i have to sleep in the weird bed?" i promise her no. we lasted that one night in the trundle bed room and actually ended up changing resorts so she could have a bed.
again, this was just our personal experience...maybe your child may not mind it. but mine couldn't sleep at all on it and i wish they'd just bring back roll away beds.
geek