Ashlee Simpson with her son while on vacation in Kailua, Hawaii. Credit: Splash News and Pictures

Ashlee Simpson is one hot single mom!

Ashlee, 28, showed off her lean, toned body in a tiny string bikini while playing on the beach with son Bronx, 4, and a pal on Saturday, Dec. 29 in Hawaii. The mother-and-son duo held hands as they splashed around in the water together and soaked up the sun.

They were also joined by sister Jessica Simpson's BFF CaCee Cobb and her new husband, Donald Faison. Cobb, 35, and Faison, 38, tied the knot on Dec. 15 with the whole Simpson family in attendance.

Jessica, who is expecting her second child with fiance Eric Johnson, tried to hide her baby bump while serving as maid of honor in the ceremony. The Fashion Star mentor, 32, finally confirmed her pregnancy on Christmas Day by sharing a photo of daughter Maxwell, nearly 8 months, sitting on the beach with "Big Sis" written in the sand.

Johnson, 33, was photographed going for a bike ride with the Scrubs actor Saturday, but the second-time mom-to-be apparently didn't tag along.

The pair, engaged since November 2010, recently put an offer on a house in Hidden Hills, California, for their growing family. And the NFL pro is also settling into domesticity. "He is amazing with Max," a source told Us Weekly. "Eric tells Jessica he has everything he's ever wanted."

A source also told Us that Simpson "is super in love" with Johnson. "Jessica is so content right now and really into family life with Eric."