This young, honestly, PLAY is important. Very important.
I would not want to enroll in anything that led to a curtailment in available time for play and age-appropriate development in other (non-cognitive) domains.
Even if you're right and readiness is there, the sooner you introduce formal instruction, the sooner the gap widens to the point that you begin to be pressured to find an alternative when your child outstrips the rate/content faster than peers (even those older).
Personally, the longer you can delay having to deal with that asynchrony gap in a formal learning environment (that is, handwritten expression limited by motor development, literacy limited by emotional readiness for YA content, etc.) the better.
Unless your child is truly YEARNING for a formal setting, I'd hold off and/or seek something play-based.
