Question: What I don't get is that if Thalia's tree had the barrier active all along while Luke, Annabeth, and Grover were kids, in the first film, how was the minotaur able to approach close to the entrance of the camp without being prevented?
Answer: Because the barrier prevents the monsters from entering the camp, not from getting close to it. It's the reason why after Percy used the fleece to strengthen the barrier, Luke and Kronos' army used the labyrinth to bypass it.
Chosen answer: Thalia's tree was poisoned, weakening it, and leaving the camp vulnerable to attack. The quest for the Golden Fleece was to provide a cure.
raywest ★