FCPS does not consider the testing from pvt psychologists, but they do from GMU. I think they use grad degree students, under the supervision of professors. I can't remember exactly, but this may be done through the Psychology Dept? I mention this because we felt more comfortable because FCPS must provide services and that is tough in this economy.
This service does cost money.
Since your child is so young, you may want to look into preschools that the county provides for kids who are suspected to have issues.
GOOD LUCK