A heartless thief is believed to have crashed a fund-raiser and made off with a bag of cash meant to help a New York City firefighter pay for life-changing surgery for his 9-year-old son.

But feisty little Aidan Sullivan -- who was born with a facial defect and no right ear -- yesterday put up a brave front, with a message for the crook: "I'm going to kick your butt!"

"I want to look normal," said Aidan, whose father, Tim, is a firefighter in the Bronx.

The third-grader has hemifacial microsoma, in which one half of the face doesn't develop correctly.

Last weekend, family friend Peter Drake, a Ridgefield, Conn., firefighter, hosted a fund-raiser, collecting between $8,000 and $9,000. But when the party at a Danbury, Conn., Irish cultural center was over, the money had disappeared.

"At the end of the night, all the money that was donated was put in a zippered bag," said Tim Sullivan. "A bartender gave the bag to Pete... He had it in his hands. He put it down to go do something, and when he came back, he saw that it was missing."

Sullivan said his longtime friend -- who has had fund-raisers to pay for Aidan's 10 previous surgeries -- is "devastated."

"Pete was so upset. He kept saying, 'I let Aidan down, I let Aidan down,' " Colleen Sullivan, 40, recalled.

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