"Happy Birthday Big Boy! Finally hit the double digits!" the Ray Donovan stud captioned his Instagram pic, which show him and Watts cutting the birthday cake. Watts also shared the same image on her Instagram Story, as well as a clip of her son enjoying the party's entertainment.
Earlier in the day, Watts also wished her son happy birthday via Instagram, writing, "His love and understanding of nature is everything. So lucky to be his mummy ❤️ such a ?."
The former couple, who split in Sept. 2016 after 11 years together, had reunited just days earlier at San Diego Comic-Con, taking Sasha and his 8-year-old brother, Kai, along for the ride.
ET spoke with Schreiber at the convention about his latest projects and then he and his sons -- Sasha dressed as a Star Wars character and Kai as Harley Quinn -- had some fun in ET's photo booth. See their amazing pics in the video below.