"It was such a magical moment," a source tells PEOPLE about the new song Etheridge wrote and performed for her new wife

What a great birthday present!

Two days after they both turned 53, Melissa Etheridge and partner Linda Wallem married on Saturday night at the San Ysidro Ranch in Montecito, California, at the foot of the Santa Ynez Mountain range.

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“They were so happy,” says a guest. “It was really beautiful.”

In the course of the intimate outdoor ceremony, Etheridge sang Wallem a new song she wrote especially for her and which Etheridge had never performed before – not even to Wallem.

“It was such a magical moment,” says the guest, who added that Etheridge wore a grey suit, while Wallem was in a white dress.

All four of Etheridge’s children participated in the wedding: Bailey Jean, 17; Beckett, 15; and twins Miller Steven and Johnnie Rose, 7.

Among those also looking on as Etheridge and Wallem exchanged their vows were Jane Lynch, Chelsea Handler, Rosie O Donnell, Whitney Cummings, Peter Facinelli and recording artist Sia.

“The ceremony was such a beautiful start to the night, says the guest. “The setting was perfect. It was something Melissa and Linda wanted early on. It was as perfect as they envisioned.”

Sunday morning, the newlyweds will leave directly for their honeymoon. Working closely with a personal vacation advisor, Etheridge planned to surprise Wallem at a secret tropical location.

The singer and the Nurse Jackie creator had been friends for more than a decade before they began dating in 2010. Last June, they announced their engagement following the Supreme Court’s historic decisions regarding gay rights, with Etheridge calling Wallem “my true love.”

“I look forward to exercising my American civil liberties,” Etheridge wrote on Twitter after the Defense of Marriage Act and California’s Proposition 8 were deemed unconstitutional. Those liberties, she added, included “getting fully, completely and legally married.”