Many newer cribs convert from a crib to a toddler bed, using the same small mattress for both options. The toddler bed design usually has a built-in bar that prevents the child from rolling out of bed. In fact, adding a bed rail to this style of toddler bed could make it difficult for the child to get out of bed. However, if your toddler’s bed doesn’t have a built-in rail, we recommend buying one. Kids aren’t likely to be injured falling out of a toddler bed, since the bed is so low to the ground. Still, just falling out can be frightening enough for a toddler and can disrupt your child’s transition and lead to other sleep disturbances.