You know, honestly, these two children were alone on the playground. Unless someone noticed them run from Mom's car in the parking lot, who would really know for sure the suspect was doing anything wrong. He's an adult, Dulce seemingly went compliantly, and he placed her in the back seat like he should (though a 5 year old is supposed to be in a carseat) and drove off. No one realized anything was out of the ordinary until later.
Also, he may have been parked along Burt St, closer to the trees in view of the basketball court. No one has said the van was parked directly behind the buildings. It would be visible from there. JMO