Mexican federal police have been stopping some minibuses and removing people from the caravan of Central American migrants, even though they had paid fares.
Associated Press journalists saw about a half dozen of the small buses pulled over at a spot near the town of Escuintla in Chiapas where police blocked the road with their cars on Wednesday. A column of other migrants on foot trekked past.
Such buses are common intercity transport in the region.
Driver Johnny Morales Castellanos said officers told him to remove the migrants because "the insurance doesn't cover them because they're foreigners."
The migrants weren't detained, but had to continue the day's planned 45-miles (75-kilometer) trek on foot in 90 degree heat — at least unless they can find another ride.
It's not immediately clear if the move is part of a broader effort to restrict the caravan's progress.
Children appear to make up only about 5 to 10 percent of migrants in a caravan traveling through far-southern Mexico. But parents' hopes for their future and fears of what could happen to them back home are clearly a motivating factor behind many people's decision to leave.
Ludin Giron is a street vendor from Choloma, Honduras. She was riding in a motorcycle taxi designed for two with her three children, as well as another mother and her daughter.
Giron held son Justin in her lap, helped by daughter Astrid, 5. Behind them sat Nicole, 3. She described the threats and pressure they would be likely to face back home once they're older.
Giron said children are always in danger from gangs in Honduras: "When they see a pretty girl, they want her for themselves. If they see a boy, they want to get him into drugs."
Refusing either can be deadly.
Beside her sat Reyna Esperanza Espinosa, a tortilla maker from Cortes, Honduras, who was alongside her 11-year-old daughter Elsa Araceli.
Espinosa said there is no work in Honduras and "that's why we decided to come here, to give a better future for our children."
The caravan set out before dawn from the far-southern Mexico city of Huixtla. Migrants hope to trek another 45 miles Wednesday to the town of Mapastepec.
It's still too soon to know exactly where along the vast US-Mexico border the bulk of this caravan will travel.
The group formed in Honduras over a week ago and only crossed into Mexico on Friday.
They may not be the only ones headed north: a US Department of Homeland Security official confirmed to CNN they are tracking another migrant caravan, as NBC first reported.
Members of the group that formed in Honduras are charting their movement daily.