“Especially with young children, the disruption strains parenting unbelievably, and when parents are strained, so are children,” said Patrick Fowler of Washington University in St. Louis, who studies homelessness and its effects on children. When families are forced to move often, he said, “kids are just constantly taking a hit on well-being, on cognitive development, on making friends, learning how to be friends, connecting with meaningful adults.”

Starting in kindergarten, public schools provide food, transportation, school nurses and relationships with teachers. And schools are required by federal law to identify and provide resources to homeless children. But low-income children under 5 have little such formal support.

Their families have constrained housing choices, given that the most affordable units — studios, one-bedrooms and single-room occupancy hotels — aren’t large enough to house children comfortably. Doubling up with friends and family is harder with children in tow. And when a family must move with an eviction filing on its record, that narrows the next housing options even more.

“​​It’s really a different section of the housing market that people are being pushed into,” said Jennifer Erb-Downward, who analyzes poverty and housing policy at the University of Michigan. This corner of the market typically doesn’t require credit reports. But the living conditions and legal protections are worse, and the rents aren’t that much cheaper. Tenants may have month-to-month leases or informal agreements. Scams are commonplace.

“We’ve trapped families into this,” Ms. Erb-Downward said.

Dionnah Wearing, a 28-year-old single mother, believed she was renting a legitimate three-bedroom row home in Philadelphia when she moved in with her young daughter in 2019. But the property manager became less responsive as problems with the house mounted, she said. Mold grew in the enclosed porch, and her daughter and a baby boy born last year developed respiratory problems.