AGrowing up as a child, in the countryside, it was really boring duringat the nighttime.

I’m assuming you are talking about the past so adding a little context to the sentence really helps the listener. In this context it’s be better to us “at” than “during”.

There was a small cabin in the southern part of the land, which ifields where the watchersmen lived.

South or southern works but southern is very formal and using south adds more a of a natural sound

DurI see so many watermelons ing the dayfield when youI pass by a watermelon land you can see many watermelons, but in the night it seemed it wasduring the day, but at night it’s so hard to find onsome.

As a child, in the countryside, it was really boring during the night.

One day we fellow kids took, some other kids and I were walking around and found a large watermelon lanfield.

你的意思你们散步的时候找到了一场西瓜田吧？你可以这样说~

There was a small cabin in the southern part of the lanfield, which iwas where the watchersmen lived.

During the day when you pass by a watermelon landfield, you can see many watermelons, but in the night it seemed it wasthey were hard to find one.

When we had just climbedOnce we set foot onto the watermelon lanfield, a bright beam of flashlight shoined onto us.

"Once..." sounds a bit better here, you could also say "Just as...". As a general rule, I don't think you can use "when" and "just" together

We knowew that although the light wasshined very clear looking from our sidely onto us, the watcherman couldn’t see us clearly from twhere he was.

We opened the watermelon with our fists and, squatted on the ground, and ate it.

This is the only time we stealole a watermelon and I am sc. Now, I wouldn't dared to think of it nowry again.

When you say "think" we don't know what you are referring to. Saying "dare" makes it clear that you are talking about stealing a watermelon.