“Brad and Angelina were heavily involved in the preparations,�? says an insider. “They mapped out and set up everything themselves.�?

Guests started arriving at the oceanfront property while Angie and Maddox were away. “He didn’t know anything about the party,�? says a Pitt family friend. “He thought he’d be staying in LA with Angelina while she worked.�?

But the next day, Maddox got the surprise of his life: a party complete with an elaborate military-style game of Capture the Flag, ATV rides, a waterslide and a dinosaur-shaped cake!

That fearlessness was on full display during the two-day birthday bash. On Aug. 4, the gang — which included about eight other boys, including the children of Angie and Brad’s security team — motored around the property on ATVs. “Playing on them is Maddox’s favorite thing in the world,�? says the Pitt family friend. “That was the main reason they had the party in Santa Barbara, so Maddox could ride around.�?

But as much as Brad wants Maddox to have fun, he always puts the boy’s safety first. “Brad was very concerned about Maddox not getting hurt,�? says the first insider. “He made sure Mad was wearing a helmet,�? as well as long pants and a long-sleeve shirt.

The festivities continued at 10 a.m. on Aug. 5 — the day Maddox was born in 2001 — as Brad set up a waterslide. “It was huge!�? says the first insider. “Brad made sure the plastic was good and wet before anyone went down. He even inflated some rafts.�?

Meanwhile, “Angelina found a comfortable spot nearby where she could cozy up with Maddox when he wasn’t sliding down the hill,�? says the insider.