The couple pledged to raise $35,000 through their Richie-Madden Children's Foundation charity in 2008 to build a playground at Los Angeles low income housing area Beyond Shelter, and are delighted with the completed project.
Despite uncharacteristically bad weather, which meant the couple's two-year-old daughter Harlow couldn't try out the new playground, the opening went well.
Nicole - who also has a five-month-old son Sparrow with Joel - said: "When we first started coming here, there were no kids outside. To see the place come to life and have a place to come outside and be kids, and have their imaginations run wild, it's really awesome."
Good Charlotte rocker Joel also announced he and his wife would be donating more money to charitable causes.
All proceeds from coffee cups designed by the Richie-Madden Children's Foundation - between now and March 15 - will go to charity, with a guaranteed minimum donation of $250,000.
The 30-year-old musician said: "We're humbled to be a part of it. Today is a big day for our family."
Joel set up the charity with Nicole when they learned they were to become parents and has previously said it just felt right to do something for disadvantaged children.
He said: "The foundation is really important to me and Nicole. We're just really happy to be a part of the community and celebrating with these kids who haven't been as fortunate as us."