I trust that there is some adult in the home keeping an eye on the children. I think they are too young to be at home by themselves. Playing with water on a hot afternoon must be tempting! Their mischief is understandable.
Obviously they did not take your beating very seriously and played the same game again. Being angry or denying a privilege may be more effective. They must understand what you say. Close off the valve that feeds water to the tap and let them have water only for toilet needs, etc. Make sure they have access to a caring adult who is within call.