This very thing happened to me too on a flight. A mother brought her child to the washroom, girl in tears needing to use the washroom soo badly. I was getting ready to take my seat, and I advised the mother that we are on the approach to the airport and it is unsafe to use the washroom at that time. She said the child would be quick, and i again advised her that it was a good idea to take her seat. The mother chose to ignore my recomendation and proceded to the washroom with her daughter. We were getting extremely close, so i knocked on the door and said through the door to stay in there, hold on tight and do not come out until i knock on the door again. We landed and after we got to the gate, I knocked on the door and the mother and daughter came out unharmed. She thanked me and apologized for putting the crew in an awkward situation. I just nodded and asked if they were ok. She said they were and deplaned.