The all-day plan is a growing trend as districts grapple with more rigorous curriculums for even the youngest students while also working within tighter budgets - the biggest barrier to expanded days.
In New Jersey, nearly 30 percent of districts still offer only half-day kindergarten. But with the state issuing beefed-up curriculum standards for next year, many experts say a full day is essential.
Dominic Gullo, professor of early-childhood education at Drexel University and a national expert on kindergarten and preschool, is in the more-is-better camp.
"All of the research is pretty clear that if implemented appropriately, full-day kindergarten is beneficial to children, beneficial to teachers, and beneficial as well to families."
Gullo said full-timers score better on standardized tests in literacy, language, and math. Teachers also rate their social competence much higher.
With a longer day and fewer students, teachers get to know the children better and can individualize curriculums and take more time with the material, Gullo said.
"Teachers feel less pressured," he said. "They have more opportunities to allow things to flow at a natural pace."
The best programs do not cram in more material and should not be mistaken for a mini first grade.
"If children are not developmentally ready to do something, regardless of full day or half day, they are not ready to do it," he said.
For New Jersey students, the extra time may be critical next year. Under the new standards, first graders will be required to read and do math on a much deeper level, said Mary Bezanis, principal of Mark Newbie.
"If people look at the Common Core, there are greater expectations," she said.
For instance, there is a new emphasis on reading nonfiction so children will be ready to read more challenging works in high school and college. That means even kindergartners will have to start learning about glossaries and tables of contents, she said.
In Camden, Gloucester, and Burlington Counties, 5,140 children attend half-day classes and 8,314 are in full-time programs. The numbers for the state are 22,040 and 70,442, respectively.
Ten states, mostly in the South, require districts to provide full-day kindergarten.
Supporters of full day include the New Jersey Education Association and the New Jersey Principals and Supervisors Association. But even Wagner, a former teacher, admitted the bill doesn't have much of a chance in a state whose Republican governor has limited a district's ability to raise taxes by imposing a 2 percent annual cap. Additionally, Gov. Christie has supported the expansion of charter schools and openly battled the state's largest teachers' union.
"This problem isn't going to be solved overnight, I know that," she said. "We can't keep saying this isn't possible. The financial burden is on working-class and middle-class families."
Those families end up paying for private after-school care, which costs up to $8,000 per child, she said.
State and federal governments need to defray the costs of the full-day programs, which can add hundreds of thousands of dollars to school budgets, said Wagner, who introduced the bill last month.
"We all have to work together," she said, adding that she planned to discuss the issue with U.S. Rep. Rush Holt, a Democrat representing the greater Princeton area, who is a member of the House Education Committee.
"Multiple studies show that the investment in kindergarten and prekindergarten pays off for society," Holt said in a statement. "But with the current mentality of cutting educational services and other investments in the federal budget, this is not an encouraging time."
A spokesman for the governor's office said he could not comment on the bill at this early stage.
Proponents of half-day programs argue that a full day in school could be too long for a 5-year-old and that longer days are too costly.
The affluent Haddonfield School District, right next to Collingswood, has never considered expanding its half-day offering, Assistant Superintendent Michael Wilson said.
"It meets the needs of the students in this community," he said. "If you look solely at student achievement and maturation, our kids do really well."
Families, too, seem to be happy with half days he said, with many paying for their own after-school programs or offering their own activities and enrichment.
However, if the state wanted to fund a full-day program, Wilson said, the district would be all for it. It would need to hire eight teachers - at roughly $70,000 each for salary and benefits, and "that's a big number," he said. Another problem is finding space.
"We'd need eight additional classrooms," he said, "and we're maxed out."
The district would also want flexibility so the program fits the needs of families, Wilson said.
For Kelly Castle, a stay-at-home mom, Collingswood's all-day program couldn't have been a better fit for her family.
"They're instilling a love of reading and they're already doing math," she said. "It's not just about going to school. They're actually learning."
Contact Kathy Boccella at kboccella@ phillynews.com, 856-779-3812, or follow @kathyboccella on Twitter.