Assuming her room is child proof, why not just try it with the crib mattress on the floor tonight? You can even go in and lay "next" to her (i.e you lay on the floor, next to the crib mattress) and explain to her that big girls sleep in a bed without a crib and this is the first night she is going to try it, that you'll lay with her while she gets comfy, but then her job is to sleep all night and if she does, she can get a new big girl bed. If it fails, back to the crib and other options
