How to handle a situation where a parent disrespects a private sports instructor.?

I am a private fastpitch pitching instructor and have been doing so for the past 5 years, running my own program with 30 kids. Recently, I had made a choice decision to cancel some lessons due to the cold weather. It was 50 degrees but with the extremely wild chilled wind, it felt much colder. I hate to even use weather as an excuse to cancel but we were hitting 9mph winds which is highly unusual. Girls were getting wind burns, fingers turning purple, loss of feeling in extremities, and also pulling muscles all while getting dirt blown in their face from the field. Mind you, these girls are about 7-11 years old and when pitching for softball, it is almost impossible to pitch with a jacket on. I thought it was in their best interest and well-being to cancel but one parent disagreed and continued to argue, insult, yell over the phone and disregarded my decision, still showing up and bringing their child. I feel as though I should let them go from my private instruction but I also feel like it is unfair to the daughter who I coach. Any advice would be greatly appreciated and also, how I should go about it.