DH and I are headed out today to get a new "big girl bed" for Liv since she is a professional crib climber. We decided to go with a full or queen mattress on the floor instead of a toddler bed. Question is, what are some guidelines I should look for in the mattress? How firm, how plush etc? Any ideas? I can't find anything online about this. She's 19 months old FWIW