Jamie Lynn Spears Birthday Bash

Jamie Lynn Spears celebrated her 17th birthday with fiancé Casey Aldridge having dinner at Ruby Tuesday and shopping at Wal-Mart, a source says."They both seemed really happy, both in really good moods," the source tells PEOPLE of the couple's outing near Aldridge's home in Liberty, Miss. "Jamie Lynn had a big smile on her face."Spears, who is pregnant with Aldridge's child, took in a quiet dinner with Aldridge, 18, at the restaurant.

"It was just the two of them," says the source. "They were like a little adult couple, very low-key."They then went to Wal-Mart, where Spears looked at sleeping bags. Jamie Lynn recently has been flashing a big ring from Aldridge. They are expecting their baby this summer.Meanwhile in Los Angeles, Jamie Lynn's big sister Britney Spears gathered with the rest of the Spears clan for a family dinner in Los Angeles.Britney, parents Lynne and Jamie and brother Bryan joined a group of about 10 people for an Italian feast at Ristorante Peppone in Brentwood.