The services in question mainly include home visits for behavioral therapy that many families say is crucial for children with autism and related conditions.
Advocates and parents said the fees would force some families to choose between paying for therapy and, say, making a mortgage payment.
"No parent should have to choose between paying a bill and paying for services that are deemed medically necessary," said Karen Shank of Quarryville. "Intensive intervention leads to the best future."
Catherine Hughes, who works for a health care provider in Allegheny County and whose 14-year-old son has a developmental disorder, said the estimated monthly cost would be from $400 to $700, too much for her to afford. She said the home therapy visits the state has subsidized have enabled her son to able to attend school with "mainstream" children.
Welfare Department spokeswoman Anne Bale said she did not have a figure on the average cost per family, but said officials determined that parents who can afford care should pay "their fair share."
"The people being targeted are ones that can afford it," she said. "One in four makes $100,000 a year."
Welfare Secretary Gary Alexander said no other state offers such generous services and taxpayers could no longer afford to subsidize the state's $340 million share of a $740 million federal and state program that extends health coverage to children with mental and physical disabilities.
Imposing the co-pays will save $9.4 million a year, Bale said.
At the rally, State Rep. Mike Sturla (D., Lancaster) called the move the Corbett administration's latest blow to a fast-fraying safety net.
In the last 18 months, the state has slashed programs such as health insurance for the working poor and reduced access to others, such as food stamps, saying taxpayers can no longer foot the bill.
"I doubt when the people were demanding the elimination of fraud, waste and abuse in the DPW budget, they were referring to saving money on the backs of working Pennsylvania families with disabled children," Sturla said.
Hughes and others said the agency did not disclose its plans for co-pays until August - and did so without legislative approval - giving families less than two months to prepare for the change.
Bale said last year's state budget spelled out that Alexander, under a newly implemented state law, is empowered to make such changes and cut costs. "This has been on the books since 2011," said Bale. "We do feel we gave proper notice."
Another critic of the move is the Hospital and Healthsystem Association of Pennsylvania, which represents 250 hospitals. The group believes the new policy puts "an undue burden on the hospital and provider community," senior vice president Paula Broussard wrote to state officials. "Ultimately, [it] could mean decreased access to services for children in need."
Contact Amy Worden at 717-783-2584 or firstname.lastname@example.org or follow @inkyamy on Twitter.