How can i make a water pump circuit
For something that a 12 year old budding engineer can work with, how about using an off-the-shelf wireless remote outlet like this $10 one from Amazon and hack the remote control instead of dealing with the AC power directly.

Try it out on your fountain pump. Then you can see if you can make an electronic circuit (transistors and opamps) or computer circuit (Arduino, Rasperry Pi, etc) to read the moisture sensor and control the remote control.

You can either electrically simulate the button presses or you can use one or two servos to physically press the ON and OFF buttons on the remote.

I hope this points you in a good and useful direction for your design. -Vince