Here are the transformed sentences using the possessive case:

1. The ball of the children is blue.
**The children's ball is blue.**

2. The house of James is small.
**James's house is small.**

3. The dog of Jess and Amy is black.
**Jess's and Amy's dog is black.**

4. The sandwich of the boy is big.
**The boy's sandwich is big.**

5. The bike of Jenny is pink.
**Jenny's bike is pink.**

6. The bag of my sister is cheap.
**My sister's bag is cheap.**

7. The suitcase of the hostess is heavy.
**The hostess's suitcase is heavy.**

8. The books of Jim and Paul are interesting.
**Jim's and Paul's books are interesting.**

9. The hair of my sisters is long.
**My sisters' hair is long.**

10. The skyline of New York is famous.
**New York's skyline is famous.**

11. The scooter of someone is under the tree.
**Someone's scooter is under the tree.**

12. The faces of the scarecrows are happy.
**The scarecrows' faces are happy.**

13. The car of Mr. Jones is in his garage.
**Mr. Jones's car is in his garage.**

14. The cake of the Smiths is big.
**The Smiths' cake is big.**

15. The nose of the clown is red.
**The clown's nose is red.**