NEW YORK -- Fergie and her husband, Josh Duhamel, are writing songs together, but they're for an audience of one: their son, Axl.

"We have own little home-written songs," Fergie said in an interview Monday, noting their tjree-month-old loves their "little ditties."

"We harmonize and we sing together," the Black Eyed Peas songstress said before singing part of one tune that began with "Baby you're my baby" and adding: "We can be quite corny."

She usually sings "Silent Night" to put Axl to sleep because it "sounds like a lullaby."

Fergie also said she and Duhamel play Rockabye Baby! CDs with music from artists like Bob Marley and Led Zeppelin and "mommy's rap music" from artists like Kendrick Lamar.

As for curse words in rap, she said: "I have to figure out how I'm going to deal with that 'cause mommy loves her rap."

Axl is the couple's first child. She and Duhamel say they want a second child and Fergie says she wants to stop at two.

The 38-year-old singer said being in "mommy mode" is what prompted her to partner with Unilever's Project Sunlight campaign that launches Wednesday in conjunction with Universal Children's Day. She said the initiative is about making the planet a better place for children.

"I have an urge to help children everywhere," she said.

Project Sunlight encourages individuals to take steps "to live more sustainably." As part of the initiative, UNICEF, the World Food Programme and Save the Children are providing school meals, safe drinking and hygiene education to two million children worldwide on Wednesday.

Fergie said she and Duhamel are taking steps to "create a brighter future for our children" by driving a hybrid vehicle, recycling and planting an organic garden at their home.

She said she's thinking about returning to work on a solo album at some point in the future. "I've got my notebooks, I've got my iPhone and I've got ideas."

Will any of the songs she and Duhamel co-wrote be included? "You never know," she said with a laugh. "Maybe we'll put them on as interludes."