As stated in another post, a fence in a culvert will not keep out baby alligators.
Nowadays signs need to be very specific because many seem to have the attitude that "it's not meant for me". We learned this as we volunteer in parks.
I really think signs should be at every water pond in that park with a picture of alligators. Visitors come from places where they don't have these kinds of dangers and they just don't think. It also probably needs to state "no wading or swimming".
Also, why is there a beach there? To most folks a beach goes with water and are used as such. Children are naturally drawn to putzing at the water's edge when there's a beach. That beach should be removed.
It was an unnecessary tragedy.