The 48-year-old British actress and 49-year-old American actor came together for the occasion Tuesday following their split in the fall.
"Happy Birthday Big Boy! Finally hit the double digits!" Schreiber captioned a photo on Instagram of himself, Watts and Sasha at his son's pool party.
Watts, who also shares 8-year-old son Kai with Schreiber, dedicated a post to Sasha on her own account. She shared a picture of the boy from her recent trip to Africa with him and Kai.
"Happy Birthday to this beautiful boy. Sasha is 10 today. Here he is in Africa, living his dream. His love and understanding of nature is everything. So lucky to be his mummy," she wrote. "Such a [gift] #inhiselement #manofthewild."
Watts and Schreiber reunited days prior to bring their sons to San Diego Comic-Con. Sasha dressed up as a Star Wars Jedi, while Kai went as the DC Comics character Harley Quinn.
"I had no idea it was going to be this much fun," Schreiber told Entertainment Tonight at the convention before joking, "When you're with your kids, you don't have to pretend you don't think it's fun."
Watts and Schreiber called it quits in September after 11 years of dating. The actress said in the June issue of Vogue Australia that she and Schreiber are "on great terms" and doing their "absolute best for the sake of the children."