I saw many people riding with children on their laps as well as the children standing between the adult's legs and driving the
ECV. A couple of the more outrageous things I saw was an older women trying to hold a newborn while driving and another had a young child "driving" the ECV while another child stood on the back and held on to the seat!
I needed to change out my ECV because of a minor problem. The Vendor met me at Animal Kingdom. He had just stopped someone who had a child on his lap and told him he couldn't do that. The rider said "I paid for this so I can do what ever I want." He was a bit shaken up when the vendor Identified himself and said he would confiscate the ECV if the child did not get off.
I did not see any CM speak to any WDW rentals that were flaunting the no extra rider rule.