From the abstract of a new NBER WP by Chris M. Herbst and Erdal Tekin:

Using data from the Kindergarten cohort of the Early Childhood Longitudinal Study, our instrumental variables estimates suggest that children receiving subsidized care in the year before kindergarten score lower on tests of cognitive ability and reveal more behavior problems throughout kindergarten. However, these negative effects largely disappear by the time children reach the end of third grade.

Something to keep in mind next time some pol bleats about caring for children. Thankfully the damage doesn't appear to be lasting.