Two-year old Maverick Hawkins sits on a red plastic car in his grandmother's living room in the picturesque town of Nevada City, Calif., in the foothills of the Sierra Nevada mountains. His playpal Delilah Smith, a fellow 2-year old, snacks on hummus and cashews and delights over the sounds of her Princess Peppa stuffie. It's playtime for the kids of the provocatively named Facebook group "Pot Smoking Moms Who Cuss Sometimes." Maverick's mother, Jenna Sauter, started the group after he was born. "I was a new mom, a young mom — I was 22 — and I was just feeling...