If I have the right park (Alvarado park) then this appears to be a nice park in a neighborhood, bordered on all 4 sides by homes. Even if this child was buried at night I would think there would be a great chance that they would have been seen. In most neighborhoods people come and go at all hours of the day and night, shift workers, mom's who run out for milk or whatever at midnight, teenagers coming home from activities etc.
Just looking at this area on google maps it looks to be full of activity, people walking, cars parked on the roadside, etc.
It's just really strange to me that no one saw a thing.