Her parents may be in the middle of a bitter divorce, but Ashlee Simpson** is keeping the peace with her ex-husband, rocker Pete Wentz.

The "Bat for a Heart" singer, 28, made nice with her 33-year-old former love and his current girlfriend, 24-year-old model Meagan Camper, to celebrate their son Bronx's 4th birthday at the Coop, a play space for kids in Studio City, Calif.

"The party was great -- super fun," a source tells Us Weekly of the superhero-themed shindig, which was also attended by Simpson pal Nicole Richie. (Conspicuously absent from the festivities: Ashlee's sister Jessica and her family, fiance Eric Johnson and daughter Maxwell Drew.)

Simpson, who finalized her divorce from the Fall Out Boy bassist on Nov. 22, 2011, appeared to get along amicably with her ex and his love of 14 months.

Not so amicable? Her parents, Joe and Tina Simpson, who announced in late October that they were divorcing after 35 years of marriage.

"Joe arrived first and made sure to leave before Tina got there," a second source tells Us. "It was very clear they didn't want to be there at the same time and are not on good terms."

The eyewitness adds that although Joe seemed happy to see his daughter and grandson -- "he hugged them both" -- he kept mostly to himself at the party. When he left, he "was hiding as he walked down the street with a hooded sweatshirt on. He looked like he didn't want to be seen."

This article originally appeared on Usmagazine.com: Ashlee Simpson Parties With Ex Pete Wentz and His Girlfriend for Son Bronx's Birthday