I only take 18m-5 and everyone is the same rate. I see what you're saying, and it IS frustrating, but that's family child care.
Are the parents seeking a developmental evaluation? I would absolutely request that the parents ask the pediatrician. I would be willing to lose the kid over it in a heartbeat. Something is wrong. I had one kid not walking at 15m (my own son) and the pediatrician wasn't willing to let it go ONE DAY past 15m, of course, as soon as we made the appointment for the eval, he started walking.
