The siblings were allegedly abducted by their biological parents Aug. 14 in South Los Angeles. The four haven't been seen or heard from since and could be anywhere in the country.
Tiffany Bryant, 39, and David James, 53, are believed to be traveling with their children in a champagne-colored Buick Enclave SUV with paper plates, according to the California Highway Patrol.
Nina Glass, director of the nonprofit Search & Rescue San Antonio, Inc., said the children's parents snatched up the kids during a Department of Children and Family Services supervised visit.
Glass said Bryant and James left with Wyman and Willow through an emergency exit and left behind their 15-month-old child. Last week, the CHP issued an Amber Alert on behalf of the L.A. County Sheriff's Department.
The children had been with a foster family since last year after the baby tested positive for drugs in their system at birth. The foster family has now gotten help from Glass' nonprofit.
"They are the type of people that can live off grid, if that makes sense. They like to be out there in the middle of nowhere," Glass said of the parents. "State parks - we always say RV parks, rest areas, you might see them at truck stops because they're nomadic. They live out of their car. They don't really have an address."
Glass said this is the second time the couple has taken their children. The first incident happened last summer when their third child was born, and they were found two months later. They were apparently granted visitation rights again after that.
Glass said the best way to help is to spread the word and share the Amber Alert flyer to as many people.
"This is not the type of search where you can actually say 'We're going to go to this location, and we're going to start searching,'" Glass explained. "This is more of a nationwide search where everybody needs to be on the lookout for these people."