Good job — your sentences are mostly correct! Here are slightly improved versions and short explanations a 7-year-old can understand.
Corrected sentence 1: Fishing boats drag big nets across the water to catch many fish at once.
Corrected sentence 2: Fishing boats sweep the waves with large nets and scoop up lots of fish!
Why these changes?
- “A lot of” is okay, but “many” sounds a bit cleaner in sentence 1.
- I changed one “big” to “large” so the sentence doesn’t repeat the same word twice.
- The meaning and grammar were already good — these are just small improvements.
If you like your first versions, you can keep them — they are correct and easy to understand.