About a month ago on an MSP-EWR flight, a mother and her small child (2.5 - 3 years old) came from the forward part of Y to use the lav well after we had crossed the Verazano bridge (maybe about 60 seconds later). The lead FA (already angry at this woman because she insisted using the lav before we pushed back and thereby delayed the flight) told the woman to sit back down. The woman got into a small verbal tiff with the FA and the FA said "Fine, but if anything happens to you, NW will not be responsible." I have to admit the woman/kid was fast. She got back into her seat just as we passed the threshold of the runway. I think what the FA said was kind of stupid. He should have stood up and blocked the isleway. I also think they should have written up the woman, but they did not. There was no excuse to put that child in danger like that.