I will summarize some options, but use the search function to find the numerous older threads in McLean and GF preschool options.
There is the preschool on the green at GF.
McLean has Country Day (on Georgetown Pike), McLean Montessori (on Kirby near Old Dominion), brooksfield School (rents from an Episcopal church on Kirby only thru June 2025 and then is moving to rent from Lewinsviille Presbyterian on Chain Bridge Rd), Lewinsville Montessori (on Great Falls St; they rent their space from the county), Westgate Children's center (also rents space from the county and is in same building as Lewinsville Montessori). Trinity Methodist on 123, Redeemer Lutheran on Chain Bridge at Westmoreland.
I think Langley School (private; not the public HS) starts at age 4.
There are sime options in N Arlington and Tysons/Vienna, but not sure of the current list.