Gwen Stefani and Blake Shelton Celebrate Her Son Apollo's 2nd Birthday With Plenty of PDA

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Blake Shelton is already tight with his girlfriend Gwen Stefani's children.

Stefani held a cowboy-themed early birthday party on Sunday for her youngest son, Apollo, who turns two on Feb. 28. Shelton was present at the big bash held at Stefani's Beverly Hills, California home, and according to an eyewitness, the couple was openly very affectionate with each another.

"[They had their] arms around one another, and [gave each other] little kisses on the cheek," the eyewitness tells ET, adding, "Blake was very hands-on with the kids, playing and walking around with them."

WATCH: Gwen Stefani Says She's Only Had Two Boyfriends in Her Entire Life

Other celebrity guests at the four-hour party -- which included a petting zoo, pony rides, and a live band -- included Jessica Alba and her husband, Cash Warren, and their two daughters, 7-year-old Honor and 4-year-old Haven; Rachel Zoe and her two sons, 4-year-old Skyler and 2-year-old Kaius; and James Corden.

Zoe Instagrammed an adorable photo of her sons at the party in matching jean shorts and straw hats, writing, "Sunday strolling and snacking #brotherlylove #happiness #familytime."

One notable absence at the lavish shindig? Stefani's ex, and the father of her three children, Bush frontman Gavin Rossdale.

Still, the party was obviously a happy occasion.

"Overall, the mood was happy and festive," the eyewitness says. "Everyone had a good time."

Stefani, 46, looked as stylish as ever at her son's birthday party, sporting a colorful sleeveless dress and putting her blonde locks into a chic ponytail. In keeping with the cowboy theme -- or you know, his usual wear -- Shelton, 39, sported a casual flannel.

The couple's Sunday celebration is just the latest outing in which the two have comfortably showed PDA. The pair celebrated Valentine's Day together at Clive Davis' Pre-GRAMMY Gala and Salute to Industry Icons honoring Irving Azoff at The Beverly Hilton Hotel, and couldn't look more in love. Stefani sat on Shelton's lap during the performances, while the country crooner happily wrapped his arms around her.

WATCH: Blake Shelton and Gwen Stefani Get Cozy at Pre-GRAMMYs Gala

Last month, Shelton told paparazzi that although the pair is really happy with one another, they're far from taking the next step in their relationship. The two Voice coaches confirmed they were dating last November.

"I think it's awfully early to be talking about that, don't you?" he said, when asked if he was going to start a family with Stefani.

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