Play date or playdate is an expression primarily used in the US for an arranged appointment for children to get together for a few hours to play. It should not be confused with dating as there is not an implied romantic component to it.
Play dates have become common because the work schedules for busy parents, along with media warnings about leaving children unattended, prevent the kind of play that children of other generations participated in. Play dates are also arranged by destinations that feature child-friendly programs like museums, parks, or playgrounds. The intention of a play date is to give children time to interact freely in a less structured environment than other planned activities might provide. Play dates are different from organized activities or scheduled sports, because they are not usually structured. Play dates are becoming part of the vernacular of popular culture and form a part of children's "down time". Most parents prefer children to use these hours to form friendships by playing with other children either one-on-one or within small groups. When children are very young, most parents stay for the play date and use the time to form their own friendships and parental alliances.
The dictionary definition of playdate at Wiktionary