The pair are currently embroiled in a bitter divorce battle but according to People, they put on a united front for the sake of their three-year-old girl.
Bethenny and Jason - who was accompanied by his parents Carol and Bob - arrived separately to the party held at Dylan's Candy Bar on the Upper East Side of Manhattan.
"Bryn was very happy to see them," a source told the magazine about the little girl's reaction to her dad and grandparent's arrival.
While Bethenny and Jason were both in attendance "they did not interact" added the onlooker.
The celebrations - which included cookies and cake - took place four days before Bryn's actual third birthday.
As RadarOnline.com previously reported Jason is agreeable to joint custody of Bryn but the former THE Real Housewives of New York star is fighting for sole custody.