Original sentence
A walrus uses its tusks to climb onto the ice and defend itself.
Five rephrased versions (each two words shorter)
Walrus uses tusks to climb ice and defend itself.
A walrus uses tusks to climb ice and defend itself.
Walrus uses tusks to climb onto ice and defend itself.
A walrus uses tusks to climb onto ice and defend itself.
Walrus uses tusks to climb ice and defend itself.
Notes
Each version is two words shorter than the original sentence and keeps the meaning kid-friendly.