Children at play brings to mind a picture of a certain type of behavior. A 'playful' behavior. Children laughing and having fun would fit this type of behavior. Adults laughing and having fun would also fit this type of behavior. Someone at play is able to amuse themselves.
Many times play involves more than one, but all involved exhibit the same type of behavior.
Recreation, though very closely related to play, might be better expressed as an activity. We participate in recreational activities. The body or mind is refreshed from recreational activities. Children at play can participate in a recreational activity. Recreation activities may be either passive or active in nature.