Must an adopted girl be told the truth that she is not the biological daughter, especially since she carries the names of her adoptive parents?

You have done well in raising the child and being good to her, but you have erred in attributing her lineage to yourselves, as this involves a religious transgression and dangers. It is obligatory to restore her lineage to its true origin and to inform her of this at the appropriate time. If you breastfed her five full sucklings during the first two years of her life, then you are her mother through rada'a (wet-nursing), and your husband is her father through rada'a.