The only requirement that is absolutely necessary for becoming a preschool teacher is the obtainment of a high school diploma or GED.
Some states and institutions require that a person has completed a minimal amount of units or credits at a university or community college--usually only 12 units.
Some institutions require an applicant obtain a certification for teaching young children. Acquiring the certification involves the completion of a few college courses (usually three to five courses) that focus on early childhood development, and the completion of several volunteer hours within a childcare setting.
Most institutions do not require that a Bachelor's degree in childhood development is obtained; however, there is not a lot of room for promotion within the preschool setting, so a degree can aid in the advancement of a teacher into a more administrative position.
It is not a requirement prior to being accepted for a job as a preschool teacher, but once hired it is required that a preschool teacher is trained in CPR and first aide.