Hilary Duff: Seeing Luca Thrive on Breastmilk Is Rewarding

"It's such a wonderful bonding time," says Duff. "Just to have your baby thrive and knowing that you're doing that and you're responsible, it's such a rewarding feeling."

New mom Hilary Duff is splitting the parenting with Mike Comrie, but there’s one aspect of tending to their son Luca Cruz that she has to handle herself.

“He’s really hands-on during the day,” the actress and singer, 24, who is the new spokesperson for Johnson’s Baby Cares initiative, tells PEOPLE of her husband.

“But at night, I’m the only one that can feed the baby. I wasn’t going to make him suffer just because I was having to get up.”

Though she leaves pumped milk for Comrie and her mother, who watches the baby while she takes a Pilates or spin class, Duff enjoys the time she spends nursing her 4-week-old son.

“It’s such a wonderful bonding time,” says Duff. “Just to have your baby thrive and knowing that you’re doing that and you’re responsible — it’s such a rewarding feeling.”

That same sense of responsibility since she entered into the world of motherhood motivated Duff to partner with Johnson’s Baby Cares for their Save the Children campaign, which will assemble, donate and distribute 5,000 Johnson’s Baby Care Kits that will provide families with baby care essentials immediately, during and after disasters.

“I’m involved in many different charities, but just becoming a new mom and when I heard about Johnson’s doing this, I was really inspired,” she explains.

“It’s such a great thing to have these kits be prepared for these new moms [so they are able] to take care of their babies. That’s all that’s been on my mind. The second that you have your baby, you’re like, ‘Okay, I’m 100 percent responsible for this.'”

At this point in Luca’s life, Duff says the relatively monotonous daily routine is broken up by frequent milestones.

“The day is exciting if he makes a new face or he gets to take a real bath instead of a sponge bath,” she shares.

“You know what was a big milestone? The belly button falling off. That was a huge one. And then I wanted to keep it but my husband was like, ‘No, that’s gross, you can’t keep it!’ I’m like, ‘I’m totally keeping it,’ and I secretly kept it.”

Duff adds, laughing, “But now it’s not a secret anymore. It’s in a Ziploc bag in the back of my makeup drawer. Luca’s going to think I’m a freak!”

— Julie Jordan and Liz Raftery