Grigorieva told journalists she already has funded operations for 50 children with heart conditions and leukemia in Ukraine, Belarus and Russia.
"Children's pain hurts me too," she told a news conference. "I always wanted to help children through music."
Grigorieva and Gibson split this month after a three-year romance. She said they had agreed to raise their baby daughter Lucia together.
She denied media reports that Gibson asked for a DNA test to prove his fatherhood.
"It's not true," she said. "There has been a lot of lies written about us."
She also said she would remove a hammer-and-sickle tattoo on her ankle, which Gibson had advised her to get.
After announcing his relation with Grigorieva last year, Gibson filed for a divorce with his wife Robyn after 28 years of marriage and seven children together.
Grigorieva also has a son with actor Timothy Dalton.