She didn't notice her daughter was gone until the little one came back without her. I guarantee she was sitting in the car on her phone, perhaps glancing up once every few minutes to check on them. It sounds like the girl was convinced to follow her abductor so it probably took a moment or two to approach her, talk to her, gain her trust and then walk back with her to the van. The mother apparently saw none of that.
Has anyone heard who provided the detailed descriptions of the man, vehicle and how he lured her? It couldn't have been the mother, since she wasn't paying attention. The little brother wouldn't be capable of those kinds of details.