Create the following items and remember to drop them right on top of the egg.

Round 1: Heater
Round 2: Hot Water
Round 3: Oven

Other potential answers: Fire

Scribblenauts Remix 7-7 Cheats Hints gallery:
7-7-1-a: Hatch it before mommy returns!
7-7-1-b: Warm the egg without getting close to it.
7-7-1-c: Place a fire near the egg.