Here is an Arduino robotics kit you can use in your pool or tub and let your children program it. The Waterbot kit teaches users about basic robotics, hydrodynamics, and programming. It has ultrasonic sensors to roam autonomously. It has an Android app for easy control.
The Waterbot is driven by an Arduino Nano. It can be programmed with the Arduino IDE or Scratch. The full kit comes with 1 Bluetooth module, LittleBot control board, 3D printable STLs, battery pack, paddles, body, and everything you need to get started.