In Dexter Season 5 Harrison's nanny looks like an important part of the story. She is the only one who passed the nanny interview held by Debra. She really even cares for Harrison. So why did she disappear in the start of season 6 ? I am unaware if it's mentioned later.
The Wikia page for Sonya also adds
According to IMDB, she took up another role Corp & Anam after season 5, so perhaps she was only contracted for 1 season and couldn't be obligated into another season.