"Twenty-one dollars an hour does not reflect the value of educators' work, their professional qualifications or the value of the children they educate," United Voice assistant national secretary Helen Gibbons said in a statement.
It's unclear how many parents, especially women, could be forced to take the day off work because of the action, the Australian Chamber of Commerce and Industry said.
"Anyone who's ever been a parent with a child in childcare would know that making alternative arrangements can be complicated and difficult, and quite often the option is to take the day off," spokeswoman Jenny Lambert told AAP.
United Voice pinned the walk-off on the federal government's failure to meet a February 1 union-imposed deadline to deliver funding for equal pay.
The action also follows last month's rejection by the Fair Work Commission of an application for a 35 per cent pay rise for childcare workers.
In a statement, Education and Training Minister Simon Birmingham said he expected early learning and childcare centres to pay workers "as much as they can afford".
"The role of government is not to run those centres but to help families access affordable care," he added.