Maxwell and Ace are the 33-year-old singer's two children with fiancé Eric Johnson. The couple became engaged in November 2010, and plan to marry in 2014. Daughter Maxwell was born in May 2012, and son Ace was born in June of last year.
"Now I know what my life is about," Simpson captions the photo of Ace. "I am so grateful."
"Maxi can't wait for RIO2," she says of her daughter in a different picture.
The proud and happy mother has been a spokeswoman for Weight Watchers since 2012. She also stays busy with her fashion line, the Jessica Simpson Collection.