I would suggest trying a capacitive sensor. They can be constructed many ways. Google for "capacitive water level" and you'll get a bunch of hits that are relevant. Not sure if you'll be able to build one that is sensitive enough for your needs, but it's a place to start.
The circuit is an emitter follower with the base voltage controlled by a potential divider formed between the soil resistance and R2 (120k). R1 sets a lower limit.
Q1: "would it work well with this transistor Q1 (c1815)?"
Yes. The transistor, c1815, is a general purpose NPN type with a minimum gain of 70, maximum of 700 and saturation of about 0.1V, max collector current 150mA so it would be quite happy in the new circuit.
In the original circuit the input voltage could go up to about 8.9V. This gives a maximum emitter voltage of 9 - 1.5V (due to the LED) = 7.5V in saturation. Three LEDs would require at least 4.5V and if we assume a current of 5mA across R6 (470R) that would add another 2.35V giving a total of 6.85V, so all three LEDs could be lit.
In your modified circuit. The maximum output voltage would be 4.35V (4.95 - 0.6) This might allow 3 LEDs to be powered if they were chosen for a low forward drop voltage (say 1.4V, giving a 4.2V drop total). Its a good range for the ADC.
Q2: Would I want to keep LED D1?
You could if you wanted to. You would lose 1.6V across the LED and transistor. Output would be between 0V and 3.4 (max soil resistance 29k). The original circuit measured the soil resistance up to about 33k. A slight increase in the 120k value (say 150k) would give you the same range.
Best Answer
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