Well, it's been a long time since I did this and I don't know if they still honor it or not.
Once you get to the theme park if you take the delivery entrance on the South side of the park. There is a nice spot by the road you can stop, lob the kid over the fence. Now it depends on the kid's age as to how long before they find him or her or if the spunky little discoverer knows how to play up the "I'm lost" gig. You should have enough time to get to the front gate and insist that you accidentally wandered out to your car to have sex and left your kids in there. By that time, the little bundle of joy has the security team pretty fired up and they will let you right in. Theoretically, of course.
Hope that helps,
FDC
*Disclaimer* No children were harmed in the making of this message.