Use the following methods to get the cat from the roof of the house:

Round 1: Use a Ladder
Round 2: Equip Wings
Round 3: Ride a Pegasus

Easy and Funny method:
1. Place a “Force Field” next to the Cat and the cat will be pushed automatically.

2. Attach a rope to both you and the kitten, you can then drag it down from the roof.

Scribblenauts Remix 6-2 Cheats Hints gallery:
6-2-1-a: Reunite the Girl and Her Kitten!
6-2-1-b: Bring the kitten down from the roof.
6-2-1-c: Climb up a ladder to reach the kitten.