My cousin gifted us her daughters mattress (she out grew it a couple months ago) - https://www.securebeginnings.com/ I have questions. Does it go right in the crib? Does putting sheets, or a water proof mattress cover negate the whole point? We also have a normal water proof mattress that my parents bought when they gifted us the crib / changing table. Should we keep this for when the baby is a toddler?