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

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

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

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. There was a small cabin in the southern part of the field, which was where the watchmen lived.

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

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

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. During the day when you pass by a watermelon field, you can see many watermelons, but in the night they were hard to find.

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. I see so many watermelons in the field when I pass by during the day, but at night it’s so hard to find some.

When we had just climbedOnce we set foot onto the watermelon lanfield, a bright beam of flashlight shoined onto us. Once we set foot onto the watermelon field, a bright beam of flashlight shined 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 knew that although the light shined very clearly onto us, the watchman couldn’t see us clearly from where he was.

We opened the watermelon with our fists and, squatted on the ground, and ate it. We opened the watermelon with our fists, 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. This is the only time we stole a watermelon. Now, I wouldn't dare to try 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.