What age do you actually enroll your toddler to a prep school? My child is approaching 3yo on Sept. some says I should already enroll her. But I think it's too early.

I say don't. The K-12 program is really strict now and they require kids to be at the right age for each level at the start of the school year. You would not want your child to repeat prep before he can get to Grade school. You can start teaching him at home though with the basics to prepare him i.e. alphabet, numbers, colors, shapes, pattern writing, simple reading.