K.J.
I also have a 3.5 yr old son, and I know that sometimes when he is rehashing scenarios for me the tone or context of a comment may not come across. There is a strong chance that the coach was totally joking and being playful when he said it. I don't find the name "butterfingers" to be damaging or cruel. It was probably said in jest, and only once, and probably without the intention to belittle. I wouldn't bring anything up to the teacher, as it seems incredibly mild to me.