Depends on the agency. That being said, I would think that mostly what would be allowed is on-duty officers patrolling the parks dressed like any other visitor. Disney has specific policy that anyone with a concealed carry permit (including off-duty law enforcement) must check their weapons in at security. This is from the Deputy Sheriff's Association of San Diego County:
http://www.dsasd.org/dsa/detail/where_can_you_carry_your_gun
Disneyland/Disney’s California Adventure/Disney World: Concealed weapons, even those carried by off-duty police officers, are not allowed in the parks. However, a customer service representative indicated that if an officer brings his or her weapon to one of the parks, you may ask to speak with a security officer and they will provide a safe storage place for your weapon while you are inside the parks. If you are staying on property, you must leave your firearms locked up at the front desk of Disney resorts or entry to the parks and not in your room safe. Metal detectors have recently been added to the bag search area.
Other parks in Southern California do allow active and retired LE to carry, but I find it interesting that Knott's Berry Farm says that they're not allowed on their roller coasters. Seems pretty reasonable to me.