The sentence you gave is almost correct but needs a little change to sound better. The sentence is:
"X-ray fish are called this because their skin is see-through."
Here is a better way to say it:
X-ray fish are called that because their skin is see-through.
Or, if you want to be even simpler for a 6-year-old, you can say:
X-ray fish get their name because you can see through their skin.
This means the same thing but sounds nicer and is easier to understand.