Good job — these sentences are almost right!
Here are clear, correct ways to say them:
- The stripes belong to the tiger. — Or a simpler way: The stripes are on the tiger.
- Tigers are the biggest cats in the world.
You can also put them together into one sentence: The stripes are on the tiger, and tigers are the biggest cats in the world.
Why this works: both original sentences were understandable. Saying "The stripes are on the tiger" sounds a little more natural for a child. If you join two thoughts, use a comma and the word "and" between them.
Nice work!