Corrected sentence:
Big machines help people everywhere — on the road, at construction sites, on farms, in the sky, and on the water. They save lives in emergencies and help keep the city clean.
What I changed and why (easy words for a 6‑year‑old):
- Added a small comma before "and": In a list we put commas between things. Putting a little comma before the last item ("and on the water") helps the sentence stay clear.
- The rest is good: The dash (—) shows the places where machines work. The second sentence is correct and sounds good: they save lives and help keep the city clean.
If you want it even simpler, you can write it like this:
Big machines help people everywhere: on the road, at construction sites, on farms, in the sky, and on the water.
Nice job! Your sentence was already very good — I just added one small comma to make the list clearer.