Iowa State Police have charged Erin Macke, 30, with four counts of child endangerment and one of transferring a firearm to a person under 21.
Police have said the latter charge was filed because a firearm was left at home within the children's reach.
Police have further said that Macke tried but failed to make child care arrangements for her children, two aged 12, one 7 and the other 6, before leaving on September 30 for a two-week holiday.
Janet Wilwerding, a Johnston police spokeswoman, said the father of one of the children was called by his child the next day, and he tipped off police that the children had been left unsupervised.
Johnston police Lt Tyler Tompkins told Des Moines television station KCCI that Macke's babysitter options all fell through, so she left the two 12-year-old twins in charge.
'They were concerned, they didn't know, they were confused,' Lt Tompkins said of the children. 'I mean, we're only talking about 12-year-olds being the adults here.'
Law enforcement went to the residence on September 21 and called the Iowa Human Services Department, which took custody of the children.
They have since been released to relatives.