In turbotax just indicate they are your children and they lived with you more than half the year (chose 7 months or longer). Both the time after the adoption, and the time when they were foster children, count as time they lived with you for purposes of the dependent exemption, and it doesn't change how you claim them as dependents if they were foster children for a few months before being legally adopted, as long as they were in your home for both periods.
