The kids are constantly fighting ?
Let them wear the virtual bumper,
set the distance to your prefernce.
The bumper will worn them whenever they are too close to, well, everything...

Step 1: Parts :

-Arduino pro mini
-4 HC-SR04 ultra sonic distance meter
-Small breadboard
-Vibration motor
-Potentiometer
-Few 220ohm resistors
-PNP Transistor
-Battary ?
-A belt
-Some zip ties
-Cables

Step 2: Diagram

There are 4 distace meters connected together to the arduino at the vcc, gnd and trig pins.
trig pin connected to arduino´s pin 3.
each distance meter connected by it self from the echo pin to arduino´s 4-7 pins.
The potentiometer connects to 5v and gnd and its output pin to arduino´s analog pin 3.
The transistor connected to pin 3 at the gate and also connects the ground through a led and a resistor.
The Vibration motor connects the emitter for power and also connects to ground.

* Note that the pic is an old ver, i made changes on build, fllow the discription.

Step 3: The Code

#define trigPin 3 //Sonar Triger
#define echoPinL 4 //Left sonar echo
#define echoPinF 5 //Front sonar echo
#define echoPinR 6 //Right sonar echo
#define echoPinB 7 //Back sonar echo
#define Vibrate 10 //Vibrate + Led pin
#define disSelect A3 //Distance Selector analog pin

int durationL, durationF, durationR, durationB, distanceL, distanceF, distanceR, distanceB, dSelValue;

void setup() {
Serial.begin (9600);
pinMode (trigPin, OUTPUT);
pinMode (echoPinL, INPUT);
pinMode (echoPinF, INPUT);
pinMode (echoPinR, INPUT);
pinMode (echoPinB, INPUT);
pinMode (Vibrate, OUTPUT);
pinMode (disSelect, INPUT);
pinMode (13, OUTPUT);
digitalWrite (13, LOW);
}

void loop() {
dSelValue = map (analogRead (disSelect),0,1023,20,400);
Serial.print ("Distance Select : ");
Serial.println (dSelValue);
triger();
durationL = pulseIn (echoPinL, HIGH);
distanceL = (durationL/2) /29.1;
Serial.print ("Left = ");
Serial.println (distanceL);
//Serial.print ("Triger = ");
//Serial.println (digitalRead (trigPin));
delay (200);

triger();
durationF = pulseIn (echoPinF, HIGH);
distanceF = (durationF/2) /29.1;
Serial.print ("Front = ");
Serial.println (distanceF);
//Serial.print ("Triger = ");
//Serial.println (digitalRead (trigPin));
delay (200);

triger();
durationR = pulseIn (echoPinR, HIGH);
distanceR = (durationR/2) /29.1;
Serial.print ("Right = ");
Serial.println (distanceR);
//Serial.print ("Triger = ");
//Serial.println (digitalRead (trigPin));
delay (200);

triger();
durationB = pulseIn (echoPinB, HIGH);
distanceB = (durationB/2) /29.1;
Serial.print ("Back = ");
Serial.println (distanceB);
//Serial.print ("Triger = ");
//Serial.println (digitalRead (trigPin));
delay (200);

if ( distanceL <= dSelValue || distanceF <= dSelValue || distanceR <= dSelValue || distanceB <= dSelValue ) {
for (int times=1; times <=8; times++) {
Serial.println (times);
digitalWrite (Vibrate, HIGH);
digitalWrite (13, HIGH);
delay (100);
digitalWrite (Vibrate, LOW);
digitalWrite (13, LOW);
delay (100);
}
}
}

void triger() {
digitalWrite (trigPin, LOW);
//digitalWrite (13, LOW);
delayMicroseconds (1000);
digitalWrite (trigPin, HIGH);
//digitalWrite (13, HIGH);
//Serial.print ("Triger = ");
//Serial.println (digitalRead (trigPin));
delayMicroseconds (1000);
digitalWrite (trigPin, LOW);
//digitalWrite (13, LOW);
}