Many early experiences that guide brain development occur in the warm and secure relationship between a young child and her/his parents. The emotional security that young children get from their parents encourages them to explore their environment with confidence. Parents provide a sense of security by helping children manage their emotions (when they become upset) and by limiting their exposure to situations that are too much for their coping skills. This sense of security supports growth of slowly developing structures in the brain that control self-regulation and self-control.
Can a paid caregiver of 21st century replace this warm and secure relationship? Can artificial school environment provide this? No. Not all. Paid-teachers without selfless and pure heart cannot provide such security.
Then why so many parents rush to put children below age 6/7 to school? Why compromise with natural development for imparting some bread-churning skills like learning foreign language alphabets?
Why such carelessness? Why escapism from responsibilities? If at all you want your children to join pre-schools, get yourselves involved in all activities.