The sentence you provided is almost correct but needs a small change for clarity and grammar. The word 'look' should be 'looks' because the subject 'Dark grey baby beluga whales' is plural, but we want to describe them as a whole. However, since 'whales' is plural, 'look' is actually correct. So instead, let's make sure everything else is clear.
Here is the corrected sentence:
"Dark grey baby beluga whales look very different from their parents."
This sentence is grammatically correct. The adjectives and nouns agree, and the sentence makes sense.
If you want to explain this to a 7-year-old: Baby beluga whales are dark grey, which makes them look very different from their white-colored parents.