I think not everyone is interested in using WiFi mesh technology.
But it is really easy to use WiFi mesh technology with ESP8266.
Support 87 WiFi devices and 100m distance will be a big advantage for ESP8266.
You can have a look at this introduction: ESP Mesh.
Step 1: Prepare
Grove ultrasonic sensor x1
3D case x1
Step 2: Code
I uploaded a lot of codes to my github:
I burned meshserver.ino this code into X-8266
I burned meshclient.ino this code into D-duino-B.
Ultrasonic sensor is plugged into D7 of the grove shield.
In fact I can also add some LEDs， Vibration motor to X-8266. That will make the process more interactive.
You can easily transfer the information like "Mom is coming" and display it.