ZombieSquirrel brings up some good points, especially about standards of care. One underlying problem with early childhood education is the u-shaped curve present due to cost; low-income families qualify for Head Start, which is generally very good, and high-income families can afford to pay more for quality care. In the middle, we have a hodgepodge of providers, both home-based and center-based. It is difficult for parents to slog through all of the choices out there for care and education, but the good news is that there is one easy thing to look for. There is an accreditation process for early childhood education, and good preschools generally participate in this accreditation, which is done by the National Association for the Education of Young Children, the professional organization for early childhood educators. You can read more about it here:
Accreditation | NAEYC