Jon Gosselin buys a birthday cake at The Cupcake Cafe in New York City for daughters Cara and Mady on October 7, 2009. Credit:

By the looks of it, Cara and Mady Gosselin will probably never forget their 9th birthday party, set to take place Thursday afternoon at the family's $1.1 million home in Wernersville, Pa.

"It could be WWIII when I get home," Jon Gosselin, 32, tells Entertainment Tonight.

He says he is preparing for the worst because estranged wife Kate, 34, is trying to change their joint-custody arrangement so that he is only at their home from 4 p.m. to 6 p.m.

"I won't allow that," he tells ET. "She's basically saying it would be too stressful to the kids to have both of us here, like we originally planned.

"I'm just going to stay," Jon previously remarked. "I own the house, so I can do what I want."

Added Jon (who was photographed buying a cake and presents Wednesday in NYC), "She's trying to prevent me from seeing my kids on their birthday because she doesn't want to see me. She doesn't want to hear my apology. She doesn’t want to work things out."

An insider tells the new issue of Us Weekly (on newsstands now) that Jon and Kate are "both determined to destroy each other at cost, whether they have the facts on their side or not. But the real victims are the children."